Abstracts of Selected Citations in the 1970's

THE SESTRI-VOLTAGGIO LINE: A TRANSFORM FAULT INDUCED TECTONIC BOUNDARY BETWEEN THE ALPS AND THE APENNINES

Scholle, P. A.

American Journal of Science, v. 269, p. 343-359, 1970.

Abstract: The Sestri-Voltaggio line forms the geologic and physiographic boundary between the Alps and the Apennines. In addition to a marked offset, the two ranges show a remarkable tectonic divergence along this line. In the French Alps, "nappe" units (as well as flysch basins) migrate toward the south and southwest; in the Apennines, the direction is reversed. Correlative units in the two ranges were deposited in a central oceanic trough and have subsequently been thrust past each other in opposite directions.

It is suggested that this unusual divergent structural pattern was produced during the closing of the oceanic region by a pair of trenches (one in the Alpine part and one in the Apennine region) with opposed Benioff plane dip directions and connected by a trench-trench transform fault. The consumption of oceanic crust in opposite directions in the two regions (as in the Tonga-Kermadec-New Hebrides trench system today) gave rise to oppositely directed nappe movement, gravity sliding, and flysch polarity in the Alps and the Apennines, The original transform fault was entirely consumed, and the Sestri-Voltaggio line is, in essence, a "tear fault" at the edge of the two sequences of nappe units which have been thrust in opposite directions. The position of the Sestri-Voltaggio line was localized by the position of the former transform fault.

SEDIMENTOLOGY OF FINE-GRAINED DEEP-WATER CARBONATE TURBIDITES, MONTE ANTOLA FLYSCH (UPPER CRETACEOUS), NORTHERN APENNINES, ITALY

Scholle, P. A.

Geological Society of America Bulletin, v. 82, p. 629-658, 1971.

Abstract: The Monte Antola Flysch consists of alternating calcareous turbidites and subordinate non-carbonate "background" sediment. Graded units as much as 28 m thick (but averaging less than 1 m) were deposited by 11 "single events," and one-half to three-fourths of this material is finer than medium silt. Most of the calcareous sediment is shown to be a reworked and sorted pelagic ooze containing abundant coccoliths, planktonic Foraminifera, sponge spicules, and echinoderm fragments. Chert occurs as cement and replacement; calcite is the major cement and replaces sponge spicules.

Evidence for turbidity current emplacement includes repeated graded bedding, microscopic shape sorting, sole markings, Bouma intervals, reworking of Foraminifera, and burrowing from the upper surfaces of beds. All criteria indicate a very distal environment of deposition, and these units clearly show that much of the fine-grained material associated with flysch deposits may be introduced by turbidity currents and not as hemipelagic material.

Reworking of contemporaneous deepwater carbonate oozes into a distal environment of deposition, with sediment ponding yielding the observed thicknesses, and coupled with the absence of carbonate material in the background sediment, are indicative of abyssal-plain sedimentation, probably at depths below the CaC0₃ compensation level (4500 to 5000 m in modern oceans).

The presence of an oceanic basin in the northern Apennines during the Late Cretaceous has considerable significance for hypotheses on the evolution of the Mediterranean region.

DIAGENESIS OF DEEP-WATER CARBONATE TURBIDITES, UPPER CRETACEOUS MONTE ANTOLA FLYSCH, NORTHERN APENNINES, ITALY

Scholle, P. A.

Journal of Sedimentary Petrology, v. 41, p. 233-250, 1971.

Abstract: The Monte Antola Formation consists of a thick section, of calcareous turbidites deposited at oceanic depths (below the calcium carbonate compensation level). Diagenetic features of the unit emphasize major differences between shallow and deep-marine diagenesis. The relatively stable original mineralogy and the absence of an early freshwater flush in the Monte Antola sediments allowed considerable compaction to occur before cementation, yielding a characteristic close-packed texture with relatively small volumes of cement. "Early" diagenesis consisted of remobilization of unstable opaline silica of sponge spicules and radiolarian tests. "Late" diagenesis included the precipitation of separate iron-free and ankeritic dolomite generations and extensive calcite cementation. Formation of recrystallization textures (microspar and pseudospar zones and veins) was controlled by original sediment grain size and occurred after cementation, but before stylolitization. Weathering has resulted in oxidation of pyrite and associated dedolomitization (calcite after dolomite). Dedolomitization of ferroan dolomites results in the formation of ferroan calcites (maintaining original dolomite zonation) which retain ferrous iron in the lattice, not as ferric oxide/hydroxide coatings.

SOUTHERN BRITISH HONDURAS: LAGOONAL COCCOLITH OOZE

Scholle, P. A. and Kling, S. A.

Journal of Sedimentary Petrology, v. 42, p. 195-204, 1972.

Abstract: The British Honduras carbonate depositional province contains a relatively narrow, deep, back-reef lagoon in which Recent fine-grained carbonate sediments are accumulating.

Examination of the finer fractions of these lagoonal muds by optical and scanning electron microscopy .reveals that a large proportion of the sediment (as much as 20 percent) is composed of coccoliths and coccolith fragments. Eight nannoplankton species have been described. Matthews (1965, 1966) failed to describe coccoliths and other fine-grained constituents in his study of British Honduras muds because he did not petrographically examine grams smaller than 20 microns. Thus comparisons between his data and the finer muds of Florida, the Bahamas, and other areas must be undertaken with caution because of the differences in grain sizes examined.

The lagoonal, coccolith-rich carbonate muds of British Honduras which surround coralgal-pinnacle reefs provide an excellent Recent analog for a number of ancient carbonates, including the Solnhofen Limestone. They also indicate that coccolith-rich sediments need not indicate deep-water deposition or unrestricted circulation far from land masses or major hydrographic barriers.

QUARTZ GRAIN SURFACE TEXTURES FROM VARIOUS SOURCE ROCK [abs.]

Scholle, P. A. and Hoyt, D. E.

Geological Society of America, Abstracts with Programs, v. 5 (7), p. 797-798, 1973.

Abstract: Although studies on quartz grain surface textures have been conducted for more than 10 years virtually no data exists on the "original" textures of quartz grains being released from primary (non-sedimentary) sources. It is difficult therefore to accurately interpret the work of those who claim that the surface textures are produced very rapidly in depositional environments. It would be at least conceptually possible that the different environmental criteria proposed by various workers relate more to depositional regimes.

Quartz grains from untransported decompositional grus derived from granitic to granodioritic plutons, pegmatites, high-grade metamorphics, and rhyolitic volcanics have been analyzed in this study. The samples come from areas of Oklahoma, Texas, California and Nevada and reflect a wide range of source lithologies and a moderate range of climates. The most common features which characterize these grains are crystal faces and fracture surfaces with V-shaped indentations, semiparallel steps, arc-shaped steps, graded arcs, parallel striations, and bifurcating ridges and grooves. Many other features are found in lesser abundance. Most of these textures have been described previously as being characteristic of various depositional environments, especially glacial and fluvial. Clearly this conclusion is premature. Because each source-rock type is characterized by a distinctive suite of surface textures it may in fact be possible to explain the distribution of quartz surface textures in Recent sediments partially in terms of source-rock distribution, or even to recognize source lithologies from surface textures where only slight abrasion has occurred.

DIAGENESIS OF UPPER CRETACEOUS CHALKS FROM NORTH SEA, ENGLAND, AND NORTHERN IRELAND [abs.]

Scholle, P. A. and Kinsman, D. J. J.

American Association of Petroleum Geologists Bulletin, v. 57, p. 803-804, 1973.

Abstract: Cores of chalks from the Ekofisk field in the North Sea have been compared with outcrop samples of the "Upper Chalk" in southern England, Yorkshire, and Northern Ireland. Techniques used included petrography, scanning electron microscopy, and isotope and trace-element geochemistry. Although all of the chalks appear to have shared a similar initial composition, subsequent variation in degree and type of diagenesis have yielded a remarkable range of ultimate lithologies. The Chalk of Northern Ireland is extremely hard (porosities of 1-10%), has oxygen isotopic values averaging -5.6 per mil, has low Sr concentrations and shows equant, blocky calcite micrite. Samples from Yorkshire have porosities of 18%, oxygen isotopes of about -4.00 per mil, and moderately extensive blocky calcite. Chalks from southern England (Dover, Thanet, Brighton) are very soft (porosities of about 43%), yield oxygen isotoped values around -2.88 per mil, and show relatively slight recrystallization to blocky calcite. The Ekofisk chalk has an average porosity of 30% with oxygen isotope values averaging -0.42 per mil, high Sr concentration, traces of dolomitization, and common rounded crystal shapes. These diagenetic variations appear related to the extent of fresh water diagenesis. Areas of continuing subsidence (North Sea basin) were not exposed to fresh water; cementation there was a function of burial depth and associated pressure solution-reprecipitation of calcite in marine or brine waters. In areas of increasing uplift (generally greatest near basin margins), progressively greater freshwater input brought about increasing diagenetic alteration of lithologic and petrophysical characteristics. These diagenetic facies are petographically, as well as geochemically, recognizable.

ARAGONITIC AND HIGH-MG CALCITE CALICHE FROM THE PERSIAN GULF -- A MODERN ANALOG FOR THE PERMIAN OF TEXAS AND NEW MEXICO

Scholle, P. A. and Kinsman, D. J. J.

Journal of Sedimentary Petrology, v. 44, p. 904-916, 1974.

Abstract: Lammated, lacy, and pisolitic carbonate crusts have been fonad on and within some outcrops of Pleistocene limestones which rise above the modern sabkha surface in Abu Dhabi. These sediments are similar in many ways to typical caliche but have unusual mineralogies and cement textures. Internally, the pisolites are uniformly aragonitic and are cemented together by fibrous aragonite and micritic and fibrous high-Mg calcite in isopachous, meniscus, and microstalactitic textures. It is suggested that caliche crusts with unstable carbonate mineralogies, together with marine or brine associated isotopic ratios and trace element concentrations, yet showing dominantly vadose textures, can develop in marine influenced areas where vadose and phreatic pore waters have unusually elevated salinities and high concentrations of Sr and Mg. Thus, in strongly evaporitic, low relief settings such as the Persian Gulf sabkhas, supratidal vadose areas are characterized, not by "fresh water," but by hypersaline brines which allow the precipitation of cements of marine chemical character with vadose textures. Similar examples are known from Bonaire, another evaporite locality. Thus, very clear distinctions should be drawn between the terms "vadose" and "fresh water"; ancient vadose textures do not immediately imply fresh-water pore fluids. One can have "hypersaline vadose" as well as "meteoric vadose" conditions.

The Abu Dhabi pisolitic crusts with their "unusual" mineralogies (including some dolomite) appear to be quite analogous to the pisolitic facies of the Permian reef complex of Texas and New Mexico and may provide a model for their formation and diagenesis. The initial presence of aragonite and high-Mg calcite in the pisolitic and microstalactic crusts of the Permian back-reef sediments would be more compatible with the observed penecontemporaneous dolomitization than would a stable low Mg calcite caliche.

DIAGENESIS OF UPPER CRETACEOUS CHALKS FROM ENGLAND, NORTHERN IRELAND AND THE NORTH SEA

Scholle, P. A.

in K. J. Hsu and H. C. Jenkyns eds., Pelagic Sediments: on Land and under the Sea: Oxford, International Association of Sedimentologists Special Publication No. 1, p. 177-210, 1974.

Abstract: Diagenetic differences, especially hardness changes, between relatively closely spaced localities in Upper Cretaceous chalks of the British Isles-North Sea area have long presented a difficult interpretive problem. In the present study, outcrop samples of the Upper Chalk in England, Yorkshire, and Northern Ireland, as well as Maastrichtian and Danian chalk cores from the Ekofisk field in the North Sea have been examined by petrographic, electron microscopic, and isotopic methods.

Although all of the chalks appear to have shared similar initial composition, subsequent variations in degree or type of diagenesis have yielded a remarkable range of ultimate lithologies. The White Limestone of Northern Ireland is extremely hard (with porosites of 5-10%) and shows very strong soft-sediment compaction textures as well as late stylolitization. The micritic matrix is composed of equant, blocky calcite (often 'consuming' coccoliths) and yields oxygen isotopic values averaging -5.6 per mil. Samples from Yorkshire have porosities of about 18%, show some development of blocky micrite matrix, and have lesser amounts of soft-sediment compaction although stylolitization is still common. Oxygen isotopes from Yorkshire samples average -4.0 per mil. Chalks from southern England (Dover, Thanet, Brighton) are very soft (porosities of 40-45%) and show little compaction or recrystallization to blocky calcite. Oxygen isotopic values average -2.9 per mil. The Ekofisk chalk has strong porosity variations (2-43%, average 30%), little compaction but some stylolitization, traces of dolomitization, and rounded, corroded crystal shapes. Oxygen isotopic values average -0.4 per mil.

Progressive increase in fresh-water alteration from the North Sea to Northern Ireland (related to the rifling and uplift of the North Atlantic continental margins) could explain the isotopic data but presents many geological problems. The assumption of an increasing gradient of hydrothermal-meteoric recrystallization of chalks from the North Sea to Northern Ireland (an area of extensive Tertiary volcanism), again related to North Atlantic rifting, appears capable of resolving both the geological and geochemical data. The striking compaction of Irish chalks is explained by expulsion of original marine pore fluids and loading by basalts; isotopic values are compatible with the introduction of thermally driven meteoric fluids.

Mapping of petrographic and geochemical gradients in chalk sediments should yield valuable information about structural and geothermal patterns in the Late Cretaceous and Early Tertiary of the North Atlantic region. Where data is clearly related to rift trends it may be useful in the prediction of patterns of deposition and diagenesis in formerly contiguous areas such as the Canadian Atlantic shelf.

ISOTOPIC AND PETROPHYSICAL DATA ON HARDGROUNDS FROM UPPER CRETACEOUS CHALKS FROM WESTERN EUROPE [abs.]

Scholle, P. A. and Kennedy, W. J.

Geological Society of America, Abstracts with Programs, v. 6 (7), p. 943, 1974.

Abstract: Hardgrounds are surfaces of synsedimentary submarine lithification recognized by hardness, superficial mineralization, encrusting epifauna, borings, reworked pebbles, and lack of compaction features. They characterize many hiatus intervals in European chalks and individual surfaces can be traced for hundreds to thousands of square kilometers.

Porosity measurements on hardgrounds and associated non-hardground chalks provide minimum estimates of the degree of cementation involved in hardground formation. In chalks with between 30 and 50% porosity, the associated hardgrounds have only 10 to 15% pore space. The original foram-nannofossil oozes near the sediment-water interface, had porosities of ca. 60 to 80% at the time of cementation. Thus, addition of 45 to 70% cement was involved in hardground formation; direct examination of hardground fabrics often shows very large amounts of cement and poor grain orientation due to lack of compaction.

Although bulk carbon and oxygen isotopic values of most European chalks have been considerably altered by diagenesis, isotopic values of associated hardgrounds invariably are less altered (more marine values). Analysed hardgrounds have del O¹⁸ ratios of -1.5 to -2.8 per mil (relative to PDB), while associated chalks range from -2.6 to -7.0 per mil. Carbon isotope ratios of hard.grounds are slightly less positive than in associated chalks. These data imply that cementation of hardgrounds took place in a marine setting, not a subaerial one. Also, the del O¹⁸ ratios show most of the cement to be early, isotopically heavy marine calcite, rather than light, later diagenetic calcite as in associated chalks. Thus, isotopic analysis can be used to conclusively demonstrate synsedimentary submarine lithification of hardgrounds.

CHALK OF THE NORTH SEA

Hancock, J. M. and Scholle, P. A.

in A. W. Woodland ed. Petroleum and the Continental Shelf of Europe: New York, John Wiley & Sons, p. 413-425, 1975.

Abstract: Upper Cretaceous sediments are the most extensive Mesozoic deposits in the North Sea, being present almost everywhere more than 60km offshore except over inversion axes in the Southern North Sea Basin. The usual thickness is not more than 500m, but in grabens and marginal troughs it reaches 1000-1600m. North of the London-Brabant High almost the whole succession is in basinal facies. In the southern North Sea the Upper Cretaceous is mostly pure chalk, but northwards in the Central Graben the chalk becomes slightly argillaceous. In the Viking Graben chalk almost disappears and the succession is of clay, which itself becomes silty northwards. All these northward changes are more marked in the Santonian-Campanian. The clastics in the Viking Graben are probably derived from Greenland which may have had a more seasonal climate than north-west Europe.

Chalk is a distinctive limestone chiefly because it was deposited as low-magnesian calcite which is stable at surface pressures and temperatures. However, chalks undergo a consistent sequence of diagenetic changes as a result of pressuresolution and reprecipitation with increasing depth of burial. From isotopic analyses of the chalks it is possible to determine maximum depth of burial, palaeogeothermal gradients, and proximity to zones of deformation. In chalk reservoirs, oil migration into the rock can be dated because its entry stopped further diagenesis.

APPLICATION OF CHALK-DIAGENETIC STUDIES TO PETROLEUM EXPLORATION PROBLEMS [abs.]

Scholle, P. A.

American Association of Petroleum Geologists Bulletin, v. 59, p. 2197-2198, 1975.

Abstract: Examination of outcrop and subsurface samples of chalks from the North Sea, onshore Europe, the Scotian Shelf, the Gulf Coast, and the U.S. Western Interior indicate that chalks undergo very significant diagenetic changes during their postdepositional history. These changes commonly involve a major reduction in porosity. Scanning-electron microscopy, light microscopy, oxygen-isotopic analysis, and trace-element analysis have been used to outline the major factors that control the patterns of chalk alteration.

Depth of burial is the key element; burial is accompanied by loss of porosity because of redistribution of carbonate by pressure solution. The reprecipitated carbonate forms mainly as overgrowths on coccolith plates and as fillings of foraminiferal chambers. Exposure to fresh water alone has little or no effect on porosity, although it may be a factor when combined with overburden pressure. Under normal circumstances, a typical nannofossil cbalk-ooze will have 70-percent porosity at the sediment-water interface. At a depth of 1 km, porosity will be reduced to about 35 percent; at 2 km, to about 15 percent; and at 3 km, to essentially 0 percent. Thus progressive lithification of chalks (and their isotopic alteration) occurs downhole or toward areas of greater burial. Petrophysical and isotopic studies can predict maximum depths of burial, paleogeothermal gradients, and proximity to zones of deformation.

However, in areas such as the Ekofisk field in the North Sea, major quantities of oil are produced from chalks having as much as 40 percent porosity (largely primary) at depths greater than 3 km. Clearly, there is a paradox here. The answer may be that other wells in the Ekofisk area, even ones that have no traces of hydrocarbons, are "anomalously" porous, and that the entire area within this central graben of the North Sea is strongly overpressured. This overpressuring, and to degree the presence of oil, minimize the differential stresses in the rock and thereby retard pressure solution and cementation. Thus, chalk porosity can be retained to a spectacular degree in overpressured areas. Such zones could be detected by seismic methods.

Significant hydrocarbon production from chalks can occur in three major settings: (1) overpressured or oil-saturated zones where these phenomena were initiated early in the subsidence history (e.g., the North Sea); (2) areas where chalks have never been buried deeply (e.g., the Scotian Shelf); and (3) cemented and fractured chalks in several possible settings (e.g., the Gulf Coast).

CHALK DIAGENESIS AND ITS RELATION TO PETROLEUM EXPLORATION: OIL FROM CHALKS, A MODERN MIRACLE?

Scholle, P. A.

American Association of Petroleum Geologists Bulletin, v. 61, p. 982-1009, 1977.

Abstract: Chalks consist largely of stable low-magnesium calcite. Thus, they undergo diagenetic alteration different from that of more widely studied aragonite and high-magnesium calcite-bearing, shallow-marine carbonate deposits. Examination of outcrop and sub-surface samples of chalks from the North Sea, onshore Europe, the Scotian Shelf, Gulf Coast, and the U.S. Western Interior indicates that chalks undergo significant diagenetic changes during their postdepositional history. Scanning-electron microscopy, light microscopy, oxygen-isotopic analysis, and trace-element analysis outline the major factors that control the patterns of chalk alteration.

The major mechanism of chalk cementation is pressure solution and local reprecipitation. Although small variations in initial grain size, faunal composition, or clay content can lead to significant bed-to-bed variations in cementation, overall patterns of chalk diagenesis appear to be related to two main factors: (1) maximum depth of burial, and (2) pore-water chemistry. With a few notable exceptions, the porosity (and permeability) of chalks decreases as a direct function of burial depth. The exceptions include cases where: (1) oil entered the rock, reducing or terminating carbonate reactions; (2) chalks are overpressured and therefore are not subject to the normal grain-to-grain stresses expected at those depths; and (3) tectonic stresses increase solution and cementation. In areas where fresh water entered the pores before major burial, chalks show a much steeper gradient of porosity loss versus burial depth as compared with regions where marine pore fluids were retained.

Under normal circumstances, a typical nannofossil chalk ooze will have 70% porosity at the sediment-water interface. At a depth of 1 km, porosity wilt be reduced to about 35%; at 2 km, to about 15%: and at 3 km, to essentially 0. Thus, one observes progressive lithification of chalks (and their isotopic alteration) as one moves downhole or toward areas of greater burial. Petrophysical and isotopic studies can predict maximum depths of burial, paleogeothermal gradients, and proximity to zones of deformation.

In areas such as the Ekofisk field in the North Sea, however, major quantities of oil are produced from chatks having as much as 40% porosity (largely primary) at depths greater than 3 km. This appears to be related largely to the widespread overpressuring of the Central graben in that area. Other such areas of anomalous porosity in thick chalk sections should be detectable by seismic methods. Significant hydrocarbon production from chalks can occur in three major settings: (1) overpressured or oil-saturated zones where these phenomena were initiated early in the subsidence history (e.g., the North Sea); (2) areas where chalks never have been buried deeply (e.g., the Scotian Shelf); and (3) cemented and fractured chalks in several possible settings (e.g., the Gulf Coast).

CURRENT OIL AND GAS PRODUCTION FROM NORTH AMERICAN UPPER CRETACEOUS CHALKS

Scholle, P. A.

U. S. Geological Survey Circular 767, 51 p., 1977.

Abstract: Production of oil and natural gas from North American chalks has increased significantly during the past five years, spurred by the prolific production from North Sea chalks, as well as by higher prices and improved production technology. Chalk reservoirs have been discovered in the Gulf Coast in the Austin Group, Saratoga and Annona Chalks, Ozan Formation, Selma Group, Monroe gas rock (an informal unit of Navarro age), and other Upper Cretaceous units. In the Western Interior, production has been obtained from the Cretaceous Niobrara and Greenhorn Formations. Significant, though subcommercial, discoveries of natural gas and gas condensate also have been made in the Upper Cretaceous Wyandot Formation on the Scotian Shelf of eastern Canada.

All North American chalk units share a similar depositional and diagenetic history. The chalks consist primarily of whole and fragmented coccoliths with subordinate planktonic and benthonic Foraminifera, inoceramid prisms, oysters, and other skeletal grains. Most have between 10 and 35 percent HC1-insoluble residue, predominantly clay. Deposition was principally below wave base in tens to hundreds of meters of water.

The diagenetic history of a chalk is critical in determining its reservoir potential. All chalk has a stable composition (low-Mg calcite) and very high primary porosity. With subsequent burial, mechanical and chemical (solution-transfer) compaction can reduce or completely eliminate pore space. The degree of loss of primary porosity in chalk sections is normally a direct function of the maximum depth to which it has been buried. Pore-water chemistry, pore-fluid pressures, and tectonic stresses also influence rates of cementation.

Oil or gas reservoirs of North American chalk fall into three main groups: 1. Areas with thin overburden and significant primary porosity retention (for example, Niobrara Formation of Kansas and eastern Colorado). 2. Areas with thicker overburden but considerable fracturing. Here primary porosity has been largely lost but secondary (fracture) porosity provides some storage capacity and greatly improves permeability (for example, Austin Group of the Pearsall field, Texas). 3. Areas with thick overburden in which marine pore fluids have been retained, or where hydrocarbons (including biogenically generated methane) were introduced early in the diagenetic history. In these settings, primary porosity is reduced to a lesser degree than in group two, and adequate reservoir properties can be maintained to depths approaching 2,000 m (6,600 ft) (for example, the Scotian Shelf of Canada).

Continued small-scale oil and gas discoveries can be expected from these types of reservoirs in North America. The prolific production of oil and gas from North Sea chalk reservoirs will not be matched in North America unless deeply buried, overpressured chalks can be located. It is the early formation of overpressures and (or) early oil input into North Sea chalks that have preserved porosities as high as 40 percent at 3,000- to 3,500-m (9,800- to 11,500-ft) depths and provided the outstanding reservoir capacity of those chalks.

DEPOSITION, DIAGENESIS, AND HYDROCARBON POTENTIAL OF "DEEPER-WATER" LIMESTONE

Scholle, P. A.

Tulsa, OK, American Association of Petroleum Geologists Short Course in Exploration Geology Notes No. 7, 27 p., 1977.

Abstract: Major oil and/or gas production has now been established in deepwater carbonate sediments of the United States, Mexico, the Middle East, and the North Sea. These reservoirs are in two main categories: pelagic deposits and transported, redeposited, shallow-water material.

Pelagic limestone, mainly chalk, is widespread and constitutes about 70% of the carbonate sediment deposited worldwide for the past 100 million years. Pelagic limestone is not particularly susceptible to the fresh-water alteration that normally affects shallow-water carbonate. Because of its-initial low-Mg calcite composition, deep-water depositional setting, and low permeability. Rather, depth of burial is the critical factor in such carbonate sediments, as overburden stresses set up mechanical and chemical (solution transfer) compaction that reduces porosity. Physical and chemical evidence indicates that only through early initiation of overpressuring, early oil input, or lack of burial can the high primary porosities of pelagic carbonate be retained, as exemplified by chalks from the North Sea, which are prolific oil producers. Later diagenetic fracturing can lead to smaller, but still important, reservoir potential, as in the Austin Chalk of Texas.

The transported shallow-water carbonates have complex depositional and diagenetic patterns often similar to those of platform carbonates. These allodapic limestones are found as fans adjacent to platform margins or as layered turbidite complexes in trenches or abyssal plains. Unstable components make these sediments prone to leaching, cementation, and dolomitization. As in untransported shallow-water limestones, the forms of porosity preservation, reduction, or creation may be complex and difficult to predict. Yet major reserves of hydrocarbons can be found in such settings (e.g., the Tamabra production in Mexico). As exploration shifts from continental shelves to slope and rise areas, such reservoirs will become increasingly common and important.

DESIGN AND CALIBRATION OF A PHOTO-EXTINCTION SETTLING TUBE FOR GRAIN SIZE ANALYSIS

Taira, A. and Scholle, P. A.

Journal of Sedimentary Petrology, v. 47, p. 1347-1360, 1977.

Abstract: A new type of settling tube has been designed for analysis of settling velocity distribitions in sediments having diameters ranging from clay to gravel size. This tube employs two different sample introduction devices for coarse- and fine-grained samples and also features an optional sample recovery system. The output of the tube's photocell has been calibrated in terms of theoretical settling velocities of glass spheres using glass microbeads and employing simple photo-extinction theory. The results agree with those predicted by theory to within about 2% and are reproducible in most cases to within 2%. The main reason for this improved precision and accuracy is the high sensitivity of the system which allows analysis of very small amounts of sample and thus reduces the effects of grain interactions to a minimum

PALEOECOLOGICAL IMPLICATIONS OF STABLE ISOTOPE DATA FROM UPPER CRETACEOUS LIMESTONES AND FOSSILS FROM THE U.S. WESTERN INTERIOR [abs.]

Scholle, P. A. and Kauffman, E. G.

Journal of Paleontology, v. 51 (Supplement 2 [abstracts for North American Paleontological Convention II]), p. 24-25, 1977.

Abstract: Nearly 300 oxygen and carbon isotope analyses were obtained from Cenomanian to Campanian units of the Western Interior. Where burial diagenesis has been slight, whole rock and individual fossil analyses are generally similar. Where burial diagenesis has been strong, whole-rock samples are significantly lighter isotopically than associated fossils.

Virtually all samples analysed (whole rock, inoceramids, or belemnites) are enriched in light oxygen isotopes relative to age equivalent European chalks. This may indicate either very high water temperatures or abnormally low salinities within the largely en- closed Western Interior seaway. In addition, all or parts of individual units, such as the Lincoln and Hartland Members of the Greenhorn Formation, the Fairport and Sage Breaks Shale Members of the Carlile Shale, the basal Fort Hays Member and the Smoky Hill Member of the Niobrara Formation, have even lighter oxygen isotopic values (-6 to -10 per mil (PDB) for most fossils). Such large shifts from equivalent European chalk fossils (0 to -2 per mil) cannot be explained by temperature differences alone. These isotopically anomalous units correlate with low eustatic sea level stands and low faunal diversity intervals indicating that these units may represent times of influx of very low salinity waters into the Western Interior seaway. Intervening units have indications of higher, but still probably less than normal marine salinity. Combined paleontological and geochemical projects can apparently resolve both diagenetic and depositional problems by placing constraints on the possible range of variables.

PETROLEUM POTENTIAL OF A POSSIBLE LOWER CRETACEOUS REEF TREND BENEATH THE U.S. ATLANTIC CONTINENTAL SLOPE [abs.]

Mattick, R. E., Bayer, K. C. and Scholle, P. A.

American Association of Petroleum Geologists Bulletin, v. 61, p. 811, 1977.

Abstract: Analysis of CDP seismic-reflection records coupled with detailed seismic-velocity analysis and a study of sedimentary processes indicate that a Lower Cretaceous shelf-edge carbonate complex may be buried beneath the present U.S. Atlantic Continental Slope. Reef, back-reef, and fore-reef facies associated with this feature could be potential hydrocarbon reservoirs for which possible analogs are the carbonate-reef trends of the El Abra-Tamaulipas Formations (Mexican Golden Lane area) and the Edwards Limestone of Texas. Seals might be provided by relatively impermeable back-reef facies and overlying fine-grained, deeper water sediments.

Additional exploration targets are structural highs over the reef trend and traps associated with faults. Petroleum generated in basin facies of the lower slope and rise could have migrated through the slope facies to the paleoshelf edge. Although current petroleum prices do not justify exploration in such deep-water areas, these targets may be of interest in the future.

DIAGENETIC PATTERNS OF THE AUSTIN GROUP AND THEIR CONTROL OF PETROLEUM POTENTIAL [abs.]

Scholle, P. A. and Cloud, K.

American Association of Petroleum Geologists Bulletin, v. 61, p. 1556-1557, 1977.

Abstract: The chalk of the Austin Group shows striking regional variations in porosity, permeability, and trace-element and isotopic geochemistry. Porosity and permeability values are highest across the San Marcos arch, where they average 15 to 30% porosity and 0.5 to 5 md matrix permeability. These values decrease slightly to the north (into the northeast Texas embayment). In northern Mexico, the Austin and its equivalents have about 3 to 8% porosity and permeability values of 0.01 md or less. Porosity and permeability also decrease in downdip sections of the Austin from outcrop to about 4,500-m depth.

The geochemical properties follow similar trends. Outcrop studies show that samples from the San Marcos arch and Sabine uplift have bulk oxygen isotopic values in the range of -2.7 to -4.0 per mil (relative to PDB). In the Rio Grande embayment of South Texas and northern Mexico, these values have shifted to -5.0 to -7.0 per mil, whereas in the northeast Texas embayment they range from -3.5 to -5.0 per mil. In downdip sections near the San Marcos arch, the oxygen isotopic values shift from about -2.8 at the surface to about -8.0 at 4,500 m. Average strontium trace-element values for the Austin Group on the San Marcos arch are 350 to 975 ppm, whereas in the Rio Grande embayment and the northeast Texas embayment they range from 950 to 1,775 ppm.

All chalk undergoes both mechanical and chemical compaction (pressure solution and reprecipitation) when subjected to sufficient differential stress. This stress generally is induced by addition of overburden, but also can be influenced by tectonic stresses and pore-fluid pressures. The presence of fresh (magnesium-poor) water in chalk, in conjunction with elevated differential stress has been shown, both theoretically and in nature, to accelerate chemical compaction greatly. Thus, the lateral and downdip variations in the petrophysical and geochemical properties of the chalk of the Austin Group presumably reflect differences in original thickness of overburden or proximity to zones of major deformation. The noted reduction in porosity between the San Marcos arch and the Rio Grande em ayment could have been produced, in the presence of magnesium-poor fluids, by about a 500-m difference in maximum overburden between the two areas. Greater overburden differences would have been required had marine (or other magnesium-rich) pore fluids been present; less overburden difference would have been needed if differential tectonic stresses were important.

Although the isotopic and trace-element values mentioned are compatible with these conclusions, they do not distinguish among the possible explanations. The smooth shift of isotopic values as a function of present burial depth in downdip sections and of probable paleoburial depths in lateral outcrop sections, indicates that maximum burial depth is the critical factor in porosity loss or retention. Only the rate of porosity loss is affected by water chemistry. Carbon isotopic analyses also rule out vadose diagenesis as having influenced porosity reduction in the Austin to any significant degree.

Oil production from the Austin Group is concentrated in the areas of the San Marcos arch and the Sabine uplift in a belt that is parallel with the outcrop trend and that ranges in depth from 200 to 2,000 m. Cumulative production, as of January 1976, from all fields in the Austin group in Texas totaled about 25 million bbl. Production of oil and gas from chalks other than the Austin has been significant, although some of these reservoirs may include sandy, calcarenitic, or other impure chalks.

Wells completed in the Austin have a long history of production at rates far lower than initial production. Most recently drilled Austin wells have initial production rates of 200 to 500 BOPD, which decline within months to about 40 BOPD. These production histories indicate that most oil production from the Austin is from fractures. Yet, the concentration of production in areas of least diagenetic alteration, in association with the long histories of slow production, indicate that extended production is probably the result of very slow drainage of oil from the rock matrix. Artificial fracturing, a completion method used on virtually all current Austin wells, enhances both initial and long-term production by allowing shorter drainage paths through a larger number of fractures.

The best future oil and gas discoveries in the Austin and equivalent lithologies will probably be concentrated in three types of areas: (1) where the chalks may have had any type of pore fluid but have not been deeply buried (that is, between 0 and 2,000 m); (2) where marine pore fluids were retained and fresh water was excluded (in such areas, significant matrix porosity can be retained to as much as 3,000 m depth); and (3) where abnormally high pore fluid pressures have reduced effective compressive stresses. Under this condition, burial depth is no longer the controlling factor in porosity loss, and porous chalks can be found at depths from 0 to more than 4,000 m.

Other production may come from areas that have low matrix porosity but intense fracturing (as along sharp flexures or faults), or from areas of abnormal lithology (e.g., bioherms, intrusive volcanic rocks, or calcarenites).

POROSITY PREDICTION IN SHALLOW-WATER VERSUS DEEP-WATER LIMESTONES

Scholle, P. A.

Society of Petroleum Engineers, Annual Technology Conference, 53rd, Proceedings, Paper 7554, 6 p., 1978.

Abstract: Limestones can be divided arbitrarily into shallow- and deep-water types. Shallow-water limestones characteristically have complex depositional facies patterns and originally are composed primarily of unstable minerals (aragonite and high-Mg calcite), which make them highly susceptible to postdepositional alteration. Small differences in burial-uplift histories can produce radically different trends of porosity preservation, destruction, or creation as a consequence of the great susceptibility of unstable minerals to alteration by fresh (nonmarine) pore fluids. These factors make porosity prediction of shallow-water limestones very difficult. Only through careful and detailed mapping of both primary facies and mineralogical trends, coupled with equally painstaking determinations of diagenetic histories and patterns, can porosity predictions be made for shallow-water limestones.

Deep-water, pelagic limestones, on the other hand, commonly are composed of a much more stable primary mineral (low-Mg calcite) and have more uniforn and predictable facies patterns. Studies performed on Mesozoic and Cenozoic chalks throughout the United States and Europe have shown that porosity is lost as a direct function of burial depth. The rates of porosity loss vary with the pore-water chemistry and, more importantly, as a function of pore-fluid pressure. If these factors are known with reasonable certainty, then reasonable or comparable predictions can be made of average subsurface porosity of deep- water limestones. Only where such limestones have never been buried deeply, where they have had an early input of oil, or where they have elevated pore-fluid pressure can high porosities and high petroleum- reservoir potential be expected.

Scanning electron microscopy, petrography, and isotopic analysis have all proved to be useful techniques for deciphering the depositional and diagenetic histories of both shallow- and deep-water limestones.

POTENTIAL OFFSHORE HYDROCARBON RESERVOIRS IN CHALK

Scholle, P. A.

Annual Offshore Technology Conference, 10th, Proceedings, Paper 3121, p. 599-603, 1978.

Abstract: As offshore exploration moves into deeper waters, fine-grained, nannofossil- and microfossil-rich limestones (chalks) will be encountered more frequently. Analysis of petrographic and petrophysical data from onshore chalks in North America and Europe and from subsurface chalks of the eastern shelf of Canada and the North Sea indicates that such limestones can be significant hydrocarbon reservoirs if one or more of the following conditions are met.

1. Shallow burial — Chalk loses porosity as a direct function of overburden stress. Thus, minimal burial generally will preserve maximum porosity.

2. Overpressuring — Early initiation of abnormally high pore-fluid pressures by rapid deposition of low-permeability sediments significantly reduces intergranular stresses and compactional porosity loss. Early input of oil from underlying sources or early generation of methane by bacteria may have a similar porosity-preserving effect.

3. Exclusion of nonmarine (Mg-poor) pore fluids — This reduces rates of cementation. Thus, exploration should be concentrated in areas that have not been uplifted and exposed subaerially.

4. Fracturing — Chalk is generally a brittle chalk rock, and even moderate deformation can produce fractures that greatly improve effective permeability.

5. Thick pay section — Because of the uniform depositional patterns and moderately high depositional rates of Cretaceous and younger chalks, continuous sections as thick as 100 to 1,500 m are sediments present in many offshore areas.

Where one or more of these requirements are met, prolific production of oil and/or gas can be obtained. In the North Sea, for example, about 2.5 billion bbl of oil have been discovered in chalk reservoirs. Despite low matrix permeabilities (1 to 2 md), the combination of abundant preserved primary porosity, thick pay section, overpressuring, fracturing, and low-viscosity oil provides very good hydrocarbon production potential in that area.

NIOBRARA GAS IN EASTERN COLORADO AND NORTHWESTERN KANSAS

Lockridge, J. P. and Scholle, P. A.

in J. D. Pruit and P. E. Coffin eds., Energy Resources of the Denver Basin [1978 Symposium Guidebook]: Denver, CO, Rocky Mountain Association of Geologists, p. 35-49, 1978.

Abstract: Natural gas from the Niobrara Formation was discovered in 1919 at the Beecher Island field in Yuma County, Colorado, but commercial development did not commence until 1972. There are now more than 30 fields located primarily in Yuma County, Colorado and Cheyenne County, Kansas. Biogenic gas is produced from chalk with high porosity but low permeability at the top of the Smoky Hill Chalk Member of the Cretaceous Niobrara Formation at depths ranging from 900 to 2800 feet. The producing area is on the eastern flank of the Denver basin and accumulations are normally on low-relief anticlinal closures. The wells are stimulated with a foam fracturing treatment and will deliver from 20 to 300 MCFGPD. The Niobrara and other Upper Cretaceous chalks are considered to be prospective as shallow gas reservoirs over a large area of the Western Interior.

PETROLEUM POTENTIAL OF THE U.S. ATLANTIC OUTERMOST SHELF, SLOPE, RISE, AND ABYSSAL PLAIN

Mattick, R. E., Girard, O. W., Jr., Scholle, P. A. and Grow, J. A., 1978,

American Association of Petroleum Geologists Bulletin, v. 62, p. 592-608, 1978.

A Mesozoic shelf margin below the present continental slope is believed to have the potential for hydrocarbon accumulations. Of special interest for the petroleum geologist are reef carbonate deposits that may be associated with this paleoshelf edge. Carbonate reef trends of the Edwards Limestone of Texas and the El Abra-Tamaulipas Formation of the Mexican Golden Lane are chosen as analogs to infer the locations of potential reservoirs and the probable relations between potential reservoirs and seals.

Petroleum could have been generated in Mesozoic or younger basin facies and migrated essentially updip, but possibly down section, to reservoir rocks in the Mesozoic shelf margin. Other exploration targets associated with this paleomargin complex and which could have been charged by migrating petroleum are structural highs above the predicted reef buildups and closures associated with known faults.

Seaward of the Mesozoic shelf edge, potential exploration targets include stratigraphic pinchouts of Lower Cretaceous and Jurassic sediments near the base of the present slope, abandoned canyon-fan complexes, channel fills on the present rise, and inferred Cretaceous and Jurassic turbidites beneath the present rise and abyssal plain.

Although current petroleum prices do not justify the development of petroleum resources in such deep-water areas, some of these targets, especially those beneath the present slope, may be of considerable interest in the near future.

DISCRIMINATION OF DEPOSITIONAL ENVIRONMENTS USING SETTLING TUBE DATA

Taira, A. and Scholle, P. A.

Journal of Sedimentary Petrology, v. 49, p. 787-800, 1979.

Abstract: The settling-velocity distributions of more than 500 modern and near-modern river, dune, beach, tidal-flat, and turbidite sediment samples were analyzed using a photo-extinction settling tube. Discriminant function analysis was applied in order to investigate the possibility of extracting environmental information from settling velocity distributions using 65 variables that describe distribution curves. The results showed that 1) good discriminations were obtained between reference sets of river, beach, and dune sediments; 2) reliable answers were obtained through identification tests of "unknown sets" using reference sets; 3) the Triassic (?) and Jurassic Navajo Sandstone samples were mostly grouped as eolian (dune) when compared with the analyzed sets of samples; and 4) the variables useful for environmental discrimination were chiefly related to the tails of distributions. Some variables commonly used for environmental analysis (e.g., mean value and sorting) were found to be more influenced by provenance factors than by environmental factors.

DEPOSITION OF RESEDIMENTED SANDSTONE BEDS IN THE PICO FORMATION, VENTURA BASIN, CALIFORNIA, AS INTERPRETED FROM MAGNETIC FABRIC MEASUREMENTS

Taira, A. and Scholle, P. A.

Geological Society of America Bulletin, v. 90, p. 952-962, 1979.

Abstract: Settling velocity distributions and magnetic fabrics in sediments from the Pico Formation were studied in order to determine the relationships between these properties and the observed sedimentary structures, and to evaluate the processes of deposition of turbidites.

Three basic types of settling velocity frequency distributions were recognized: P1, profile, a low, flat distribution pattern indicating very poor sorting; P2, profile, a distribution which shows a distinctive mode; and P3 profile, a slope-shaped pattern composed predominantly of fine materials. P1 was found in the graded and massive divisions of turbidites; P2 was found in the lower division of horizontal stratification and the ripple stratification division; P3 was found in the uppermost divisions. Comparison of these patterns with previous results from modern sediments reveals that P2 and P3 are similar to patterns found in fluvial environments, whereas P1 was quite rare in "normal" current- or wave-formed deposits.

The uniqueness of the graded and massive division is also evident in the results of the magnetic analysis. Although the magnetic fabrics in the upper parts of turbidites show similarity to other current-formed fabrics, the magnetic fabrics of the graded and massive divisions are quite different. The magnetic fabrics in the graded and massive divisions are characterized by (1) the presence of the current-normal orientation, and (2) less foliated and inhomogeneous fabrics which are indicated by high imbrication and q-value as well as large standard deviations of q-value and Kmax directions. Comparison with results from modern sediments indicates that fabric characteristics in the sediments of the graded and massive divisions are best explained by a combination of (1) an orientation mechanism related to layer by layer grain collision in a highly concentrated flow and (2) an orientation mechanism related to the suspension of grains in a viscous flow. This evidence indicates that a highly concentrated and partly viscous basal flow in turbidity currents may be responsible for the deposition of the lower part of the graded division and the massive division, whereas a more diluted flow may be responsible for the deposition of the upper divisions.

ORIGIN OF BIMODAL SANDS IN SOME MODERN ENVIRONMENTS

Taira, A. and Scholle, P. A.

Journal of Sedimentary Petrology, v. 49, p. 777-786, 1979.

Abstract: Grain settling velocity distributions of sediments from modern fluvial channel, eolian barchan dune, beach, and tidal flat environments of the United States and Mexico were analyzed using a photo-extinction settling tube. The results revealed the occurrence of bimodal distributions in several specific places: (1) trough cross stratification in fluvial channels, (2) the lower slip face, lower stoss side, and crest of barchan dunes, (3) eolian mega-ripples, (4) lower beach foreshores, (5) beach storm layers, and (6) inner parts of tidal flats. Possible sorting mechanisms for generating bimodal characteristics of these deposits are suggested and are classified into three categories: a) Mixing of two sorting processes which have different sorting tendencies. For example, mixing of avalanche sorting and projection sorting on the dune slip face is responsible for bimodal sediments of types (1) and (2). b) Special hydraulic circumstances yielding high mobility of the coarse fraction; type (3) and (4) bimodal sands are examples. c) Unusual transportation events. Storm transportation of a corase fraction into ordinarily coarse-sediment-free environments may be responsible for type (5) and (6) distributions. This study has demonstrated that bimodal distributions of grain settling velocity of sediments could be interpreted by simple and basic sorting processes.