Abstract For the last several decades, gold exploration in Nevada has been strongly focused on sedimentary rock-hosted gold deposits in the Carlin, Cortez, Independence, and Getchell trends in north-central Nevada. Accordingly, less exploration activity has been directed toward the search for similar gold deposits in the eastern Great Basin, south and east of the major trends. Deposits in the central and northern Carlin and Cortez trends are hosted primarily in Upper Devonian middle slope soft-sediment slumps and slides and base-of-slope carbonate debris flows, turbidites, and enclosing in situ fractured lime mudstones. This is in marked contrast to gold deposits in the eastern Great Basin that are hosted primarily in three chronostratigraphic horizons: (1) shallow-water, Cambrian and Ordovician carbonate platform interior, supratidal karsted horizons and shelf lagoon strata, associated with eustatic sea-level lowstands and superjacent, transgressive calcareous shale and siltstone horizons that are deposited as sea level begins to rise, (2) Early Mississippian foreland basin turbidites and debris flows overlying karsted Late Devonian platform strata, and (3) Pennsylvanian and Permian shallow marine basin strata. Stratigraphic architecture in these three horizons was influenced in part by Mesozoic (Elko and Sevier) contractional deformation, including low-angle thrust and attenuation faults, boudinage, and large-scale folds, which in turn affected the orientation and localization of synmineral brittle normal faults. A compilation of past production, reserves, and resources (including historic and inferred) suggests an overall endowment of over 41 Moz of gold (1,275 tonnes) discovered to date in the eastern Great Basin, some in relatively large deposits. Significant clusters of deposits include the Rain-Emigrant-Railroad and Bald Mountain-Alligator Ridge areas on the southern extension of the Carlin trend, the Ruby Hill-Windfall-South Lookout-Pan on the southern extension of the Cortez trend, and the Long Canyon-West Pequop-Kinsley Mountain area near Wells, Nevada. Sedimentary rock-hosted gold deposits extend to the eastern edge of the Great Basin in Utah and Idaho and include the past-producing Black Pine, Barney’s Canyon, Mercur, and Goldstrike mines. The recognition of widespread, favorable host rocks and depositional environments on the Paleozoic platform-interior shelf in the eastern Great Basin opens up vast areas that have been relatively underexplored in the past. A basic premise throughout this paper is that the better we understand the origin of rocks and the depositional and postdepositional processes under which they formed, the more accurately we can make well-founded stratigraphic, sedimentological, structural, geochemical, and diagenetic interpretations. Without this understanding, as well as the rigorous application of multiple working hypotheses to explain our observations, the advance of science and the discovery of gold deposits is problematic.

Abstract The passive margin carbonate platform in the Middle Tien Shan rests on Givetian–Frasnian red siliciclastic strata. It evolved from an attached carbonate platform in the Famennian and early Tournaisian to an isolated carbonate platform in the late Tournaisian to early Bashkirian. The open-ocean side of the platform was reef-rimmed, whereas the continental side was both reef- and shoal-rimmed. Platform interiors exhibit low-energy facies during the Famennian to early Visean and high-energy facies during the late Visean to Bashkirian. Eustatic sea-level rises in the middle Tournaisian, early Visean and near the Visean and Serpukhovian boundary caused major reorganizations in platform architecture. Deformation in the middle Bashkirian reflects the onset of a convergent margin. Flexural loading by an orogenic thrust wedge controlled basin subsidence along the southern edge of the Middle Tien Shan in the Late Pennsylvanian to Asselian. Cessation of deposition in the Asselian followed by folding and granitoid plutonism reflects the onset of a rigid collision. Devonian to Permian carbonates represent outcrop analogues of coeval oil- and gas-rich carbonate platforms in the North Caspian basin and can be used for comparative and predictive sedimentological studies. Palaeozoic carbonate reservoir facies may host subsurface Cenozoic oil fields in the Fergana Basin.

Abstract For decades virtually all of the former USSR was closed to non-Soviet bloc geologists for conducting collaborative geologic studies. This was unfortunate, inasmuch as this immense territory houses much of the earth’s geological history and is estimated to have approximately 22 percent of the world’s known petroleum volumes ( Ronov et al., 1980 ; Klett et al., 1997 ). Outside of the Russian literature and a limited number of papers and books translated into non-Russian languages, details about the petroleum resources and the evolution of carbonate rock complexes in the former USSR have been infrequent. The Paleozoic carbonate systems of the CIS are scientifically important because they include many of the world stratotype localities, including the Cambrian-Ordovician boundary and many stages of the Carboniferous and Permian. Moreover, Paleozoic carbonates of the CIS provide a considerable record from which to evaluate the evolution of reef-building organisms through time, variations in paleoclimate, changes in global sea level, paleotectonics, and secular variation in the composition of ancient sea water, and primary mineralogy of marine carbonate precipitates. All of these variables have a major impact on the sedimentation, accumulation, and diagenesis of carbonate rocks, and thus a better understanding of carbonate systems of the CIS will advance our general knowledge of carbonate sedimentology. This Special Publication was derived from an SEPM research symposium entitled “Carbonate Reservoirs and Carbonate Field Analogs of the CIS”, which was held at the 1997 Annual Convention of the American Association of Petroleum Geologists and SEPM (Society for Sedimentary

Abstract The distribution of Paleozoic reefs in the boundaries of the former USSR has been studied within the context of the stratigraphic record and relationship to paleotectonic elements and paleogeography. Four cycles of reef building have been identified: 1) from the Early to Middle Cambrian, 2) from the Late Ordovician to Frasnian, 3) from the Famennian to Early Bashkirian, and 4) from the Late Carboniferous to Permian. Each reef–building cycle starts with the accumulation of mud mounds, then after a brief pause, skeletal bioherms and biostromes, and finally well–developed shallow–water reefs. For each cycle, specific species of reef builders are recognized and that periods of intensive reef building coincide with periods of dynamic plate movements. The Paleozoic reefs of the former Soviet Union were deposited on various tectonic elements and experienced a wide range of tectonic histories. In cratonic and peri–cratonic settings, the main structural elements are summarized as follows: marginal and intracratonic depressions, intracratonic rifts, and shelf and passive margins. In oceanic settings, the main structural elements include microcontinents and individual stable blocks, and elements located adjacent to subduction zones and zones of volcanic activity. During periods of ocean expansion, the reef–building process became more dynamic and reefs were predominantly deposited upon stable structures such as paleocratons, microcontinents, and stable outliers. During periods of collision, the deposition of reefs upon tectonic elements related to subduction zones and volcanic activity is observed to increase. It is concluded that periods of dynamic reef building correspond with periods of accelerated tectonic movements in the Early Cambrian, Middle to Late Devonian, and Permian. Reef building within the boundaries of the Former Soviet Union reached its zenith during the Early to Middle Devonian during an intense period of tectonic rifting and drifting. Moreover, it is postulated that the emergence and development of skeletal reef–building communities was a response to significant rates of subsidence, the emergence of differentiated relief, temperature shifts, and the evolution of biotic communities capable of offsetting the dynamic subsidence through high carbonate accumulation rates.

Abstract Meter-scale shallowing-upwards cycles (SUCs) are common in the shallow marine platform carbonate deposits around the world and throughout the geologic record. Despite that, their origin remains largely unresolved. Possible mechanisms capable of producing this kind of cyclicity include periodic (orbitally driven) and random sea-level oscillations, Ginsburgian autocyclicity, and random or quasi-periodic tectonic processes. Shallow-marine carbonate deposits of the Cambro-Ordovician Aisha-Bibi seamount (Malyi Karatau, Kazakhstan) contain tens to hundreds of meter-scale SUCs. Those cycles are typically composed of (from bottom to top) (1) transgressive lag flat-pebble conglomerates, (2) subtidal cross-stratified and bioturbated peloidal grainstones with mudstone drapes, intraformational flat-pebble conglomerates, and occasional thrombolites, (3) intertidal ribbon rocks (centimeter-scale heterolithic interlayering of mudstone and grainstone), and (4) upper intertidal-supratidal mudcracked cyanobacterial laminites and stromatolites. Boundaries between the subfacies within each cycle are typically transitional, reflecting gradual change from one laterally adjacent depositional subenvironment to the other; the boundaries between the cycles, however, are sharp and commonly erosional, separating non-adjacent subfacies and indicating abrupt change in sedimentation. Although each depositional cycle represents deposition under conditions of gradual lowering of relative sea level, the uninterrupted stacks of multiple SUCs indicate occurrence of repetitive high-frequency relative sea-level oscillations during the sediment accumulation. Three continuous, well-exposed cyclic sections were measured in detail; the longest one (Ooshbas) contains 124 cycles. That allowed us to test the Aisha-Bibi SUC sedimentary record for presence of orbital periodicities. Frequency modulation analysis was applied in order to overcome problems related to variable sediment accumulation rates. Time-series analysis of the Ooshbas SUC stacking patterns revealed the presence of Cambrian precessional periodicities, registering three out of four characteristic precessional “bundling” frequencies with > 90% confidence. That suggests that the depositional cyclicity recorded in the shallow marine deposits of Aisha-Bibi was driven by Milankovitch-forced eustatic sea-level changes.

Abstract The Roman Trebsa field is the best–studied example of a widespread, Lower Devonian carbonate play in northeastern Timan– Pechora Basin, CIS. Within Roman Trebsa field, the Lower Devonian is progressively truncated from east to west by a major, pre–Upper Devonian unconformity. Reservoirs consist of three stacked, dolomitized, shallow marine subtidal to peritidal intervals separated by deeper, subtidal sealing intervals. A sequence–stratigraphic framework, a reservoir zonation, as well as depositional and diagenetic models have been developed from cores and well logs of Roman Trebsa. The Lower Devonian is divisible into five sequences, SQ1–SQ5. Each sequence has been further divided into fourteen zones, based on reservoir potential. A distally steepened ramp model is proposed for the Lower Devonian. The inner–ramp complex consists of a mosaic of dolomitized, reservoir–prone, peritidal to high–energy shoal facies. Middle and distal ramp facies are muddier, more faunally diverse, and less dolomitized than the inner ramp. The deep marine environment consists of mud–supported lithologies alternating with skeletal layers interpreted as storm deposits. Syndepositional faulting may have accounted for distal steepening. Three styles of reservoir development are observed throughout the Lower Devonian. Most common is a matrix–dominant system, occurring in grain supported dolomites preserving primary interparticle porosity but also containing layers of coarse non–mimetic, sucrosic dolomite with intercrystalline porosity. Less common is a vug–fracture system consisting of layers of centimeter–size vugs with halos of coarse, dolomite, poorly interconnected by 1 mm aperture fractures. Least common is a porous, late (post–stylolite) phase of dolomite recrystallization developed along prominent faults. Differences in reservoir style are partially attributable to original depositional texture. Updip, grainy facies form the matrix dominant systems, while vugs and fractures may be more prevalent in distal, muddy facies tracts. Fault–related dolomite recrystallization may occur in otherwise non–reservoir prone lithologies. Overall reservoir quality of Lower Devonian reservoir zones is moderate, with typical core porosities in the 6–10% range and permeabilities between 10 and 100 md. Reservoir properties are fairly uniform across the field, particularly above the oil–water contact. Limited core data suggest that saddle dolomite cementation accounts for poorer reservoir quality below the oil–water contact. Many aspects of the geology of Roman Trebsa point to a range of possible reservoir descriptions. The major uncertainties in reservoir models were considered to be: (1) lateral continuity of porosity and permeability, (2) the lateral extent of high–permeability streaks (like coarse layers of recrystallized dolomite or vuggy layers with fractures), and (3) reservoir segmentation due to fractures or breccias beneath the pre–Upper Devonian unconformity.

Abstract The Kharyaga field, located in the Komi Republic (CIS), is a multi–horizon oil–bearing reservoir. The Upper Devonian carbonates of Object 2 are one of the most interesting reservoir targets. While drilling, several wells reported heavy, sometimes total mud losses, tool breaks, and falls of several meters. Cores cut in those wells were generally poorly recovered, and contained large vugs and an abundance of fractures. A detailed examination of these cores reveals the karstic nature of the reservoir and includes the following: conduits passing to large cavities, caves with network distribution, fissures enlarged by leaching, morphologies with erosional overprint, karstic breccia, local filling with green shales, geodic sparite cement, and true speleothems. All these karstic features are documented from thin–section study. Some bitumen is locally associated with the karstic levels. The karstified zone exhibits a wide vertical and horizontal distribution. The karst appears polycyclic, and one of the latest phases remains widely open. Overall porosity is increased and permeability is drastically improved by karsting. The tested wells in this zone have a very good productivity index. The reservoir characteristics of paleo–karst related breccia in another sector are also attractive when compared to the non–karsted zones. The mapping of the karstified reservoir is based on all the data collected (mud logging, electrical logging, cores, and production data) and is a critical factor for reservoir management and field development.

Abstract The Bolshoi Karatau carbonates of southern Kazakhstan record the development of a 4,500–m–thick platform that evolved close to the North Caspian Basin of western Kazakhstan during the Late Devonian and Carboniferous. Carbonate facies in the Bolshoi Karatau Mountains provide outcrop analogs for coeval reservoirs in supergiant oil and gas fields in the North Caspian Basin. The carbonate platforms in the Bolshoi Karatau and the North Caspian basin are similar in several important ways. First, both the Bolshoi Karatau and the Tengiz oil field carbonate platforms were initiated in the Late Devonian and ended in the Bashkirian, a span of about 50–55 Myr. Second, the stratigraphic thickness and facies of the Bolshoi Karatau and the Tengiz oil field are similar. Third, the proven oil reserves in Tengiz occur in the Visean through Bashkirian, and these strata are very well exposed in the Bolshoi Karatau. The seaward margin of the Bolshoi Karatau carbonate platform was probably structurally controlled by the rifted edges of a passive continental margin. The overall geometry of the carbonate platform was controlled by thermal subsidence and local tectonics. Over a 50-55 Myr period of time this passive margin underwent thermal subsidence, normal faulting, and numerous sea-level fluctuations of varying amplitudes. Sedimentation rates suggest that subsidence decreased exponentially. Sediment accumulation rates were 185-285 m/Myr during the Late Devonian, 60-100 m/Myr during the Tournaisian, 35-50 m/Myr during the Visean, 15-30 m/Myr during the Serpukhovian, and 20-50 m/Myr during the Early Bashkirian. The net result was a carbonate platform that evolved from reef and sand-shoal-rimmed platforms in the Devonian to deep-water ramps and skeletal mounds in the Tournaisian and Early Visean and to skeletal mounds and sand-shoal-rimmed margins in the Middle-Late Visean, Serpukhovian, and Bashkirian. Depositional sequences, sequence boundaries, and facies were controlled by relative sea-level oscillations, sedimentation rate, climate, subsidence, and biotic changes through time. Relative sea-level changes were responsible for shelf-margin flooding and backstepping, multiple stacked sequence sets in shelf interiors, paleosols, extensive meteoric diagenesis, and karst and collapse breccia. Five depositional supersequences (second-order cycles), of seismic stratigraphic scale, are recognized in the Bolshoi Karatau: (1) Supersequence #1 includes Frasnian and Famennian Girvanella-Renalcis algae-bryozoan-sponge boundstone and cementstone reef-rimmed facies. Shelf interiors contain skeletal mud mounds, carbonate sands, cryptalgal laminites, evaporitic laminites, and a 90-m-thick regionally extensive karst and evaporite collapse breccia. Basin margins contain carbonate turbidites and megabreccia debris-flow aprons. (2) Supersequence #2 comprises Tournaisian-Lower Visean ramps of brachiopod-crinoid biostromes and abundant tidal-flat facies. Seaward ramp settings contain Waulsortian skeletal mud mounds and bioclastic turbidite aprons. (3) Supersequence #3 comprises Lower Visean-Upper Visean carbonates that have both Waulsortian skeletal mud mound shelf margins and shoal-rimmed shelf margins. Waulsortian skeletal mud mound margins and upper and lower slopes are sponge-Tubiphytes-algal-bryozoan boundstone and cementstone. Deeper-water parts of these slopes contain carbonate turbidite aprons. Shelf-interior facies consist of interbedded ooid and bioclastic sands. Shoal-rimmed platform margins are dominated by cross-bedded ooid-bioclastic sands and turbidite aprons. (4) Supersequence #4 comprises Upper Visean-Lower Bashkirian basinal carbonate turbidites and upper-slope Waulsortian skeletal mud mounds made up of Donezella algae-Tubiphytes-sponge-bryozoan boundstone and algal rudstone. These upper-slope mounds developed in a setting that was receiving abundant carbonate turbidites and debris flows. Shelf margins and shelf-interior facies consist of interbedded ooid and bioclastic sands and phylloid algae-rich sands. (5) Supersequence #5 encompasses Lower Bashkirian Waulsortian skeletal mud mounds that formed on upper-slope and drowned-shelf-lagoon settings. Waulsortian mound facies are dominated by phylloid algae-Donezellid algae-Archaeolithoporella algae-gastropod-brachiopod boundstone and cementstone facies. The boundstone facies are typically interbedded with algal rudstone and possible pisoid facies. Drowned shelf-lagoon facies comprise lime mudstones and carbonate turbidites derived from an ooid-bearing shelf margin. In the Bolshoi Karatau, thick stacks of upward-shallowing cycles of dolomitized and karsted shelf-margin and shelf-interior ooid-bioclastic sands form potential reservoirs, whereas lower-slope and upper-slope Waulsortian skeletal mud mounds contain abundant marine cement and are relatively tight. Reservoir enhancement is related to early dolomitization and meteoric diagenesis. These geometric and diagenetic patterns are analogous to some reservoirs in the North Caspian Basin such as in the Karachaganak and Tengiz fields and the newest supergiant, the Kashagan oil field. Bolshoi Karatau studies provide important data on the heterogeneity of the reservoirs in terms of their facies types, their cyclicity and stacking patterns, the origins of these stacking patterns, porosity-enhancing conditions associated with this cyclicity, and the spatial distribution of these reservoirs in the platform. These outcrop studies should be valuable for better understanding and predicting the characteristics and development of the North Caspian Basin oil and gas reservoirs as well as the oil and gas reservoirs in the Timan-Pechora and Volga Ural basins of Russia.

Abstract Field mapping, facies analysis, and laboratory study have been completed on Upper Devonian to Lower Permian carbonates of the Bolshoi Karatau Mountains, southern Kazakhstan, and of the southern Urals, Russia. These data suggest that: (1) following the Late Devonian extinction event, major shifts took place in the composition and spatial distribution of reef biota and in the mineralogy and stable- isotope composition of abiotic marine precipitates, and (2) the distribution of reservoir-grade porosity is strongly influenced by early-marine cementation, dolomitization, and meteoric-related diagenetic processes. Corresponding shifts in the morphology of carbonate platforms, and the spatial distribution of potential reservoirs, suggest a strong biotic and abiotic influence on platform architecture, cementation patterns, and reservoir development. Importantly, results of this study describe a series of depositional models and associated diagenetic patterns that are comparable to the known distributions of reservoirs in giant carbonate fields of similar age in the subsurface of the North Caspian basin. Upper Devonian (Fammenian) reefs are composed of Girvanella boundstones, algal cementstones, and brachiopod bioherms and form reef-rimmed platforms. Boundstone and slope deposits are cemented by fibrous and radiaxial fibrous calcite; inner-shelf peritidal deposits are extensively dolomitized, and several intervals of karst are present near the Fammenian-Tournaisian boundary. Tournaisian to Lower Visean carbonates lack a defined organic margin and form ramps that contain Waulsortian mud mounds; inner-shelf peritidal facies are partly dolomitized. Middle-Late Visean and Serpukhovian carbonates form rimmed platforms consisting of peritidal shelf lithologies, grainstone margins, resedimented carbonate and microbial boundstone upper slopes, and deep slope mounds. Shelf interiors consist of cyclic, meter-scale peritidal carbonate lithologies that are capped by paleosols and contain intervals of carbonate-clast conglomerates that are interpreted as forming through prolonged periods of exposure. These layered shelf carbonates are both dolomitized and karsted, and their sedimentation and early diagenesis is attributed to climate changes brought about by the onset of glaciation during the Middle-Late Visean. Slope mounds are composed of sponge-bryozoan-Tubiphytes-algae boundstone and cementstone. Radiaxial fibrous calcite infills skeletal growth frameworks and hash layers, and forms thick linings in stromatactis cavities. Thus mound stabilization is achieved through void cementation and the mantling of mud-dominated fabrics. Bashkirian shelf margins consist of donezellid-brachiopod boundstone and cementstone; shelf interiors contain prolific oolitic grainstone. Bashkirian ooids and marine cement were dominantly composed of aragonite and were particularly susceptible to exposure-related diagenesis and indicate that shelf and margin sediments were exposed during lowstands. Lower Permian shelf margins and slope mounds are composed of bryozoan- Tubiphytes -algae boundstone and cementstone, Palaeoaplysina rudstone, and foram-rich lithologies. Incomplete infilling of boundstone and rudstone lithologies by radiaxial calcite and subaerial exposure resulted in the preservation of primary growth and shelter cavities. On the basis of detailed field and laboratory study of outcrop, it is concluded that biota, eustasy, primary mineralogy, and diagenesis exerted strong control on platform architecture and reservoir development in thick, Upper Devonian to Lower Permian carbonates. Following a Late Devonian extinction event, platform architecture exhibits a transition from reef-rimmed to ramp to shoal-rimmed to reef-rimmed carbonate platforms. This transition in platform architecture is interpreted to reflect: (1) the initial replacement of reef-forming skeletal organisms by reef- forming algae and maintenance of reef-rimmed platforms (Fammenian), (2) demise of algal-constructed margins concomitant with biotic recovery and formation of ramps (Tournaisian-Lower Visean), (3) deposition of microbial boundstone in upper slopes and early stabilization of ooid grainstone to form shoal-rimmed platforms (Middle Visean-Serpukhovian) with mound-building organisms relegated to subphotic slope positions, (4) eventual reestablishment of shallow-water benthos capable of forming wave-resistant platform margins (reef-rimmed platforms; Bashkirian), and (5) formation of reef-rimmed platforms with slope-mound complexes (Lower Permian). Marine cement is particularly abundant in slope mounds, resedimented upper-slope deposits, and reefal boundstone at platform margins, where it occurs as frame-filling cementstone and thick isopachous linings in pores. An association between cementstone and trophic biotic assemblages suggests that massive cementation was enabled by upwelling of carbonate-saturated bottom waters. Isotopic compositions of originally aragonitic and calcitic marine cement are enriched in 13 C and 18 O and are in agreement with secular trends defined by other Devonian to Permian carbonates. Thus, early marine cementation plays an important role both in constructing these lithofacies and in reducing most primary depositional porosity. As a result, porosity-enhancing diagenetic processes in the slope and platform margin (e.g., early marine- related dolomitization; leaching) are limited to partial alteration of skeletal fabrics, cements, and micritic matrix. Partial marine cementation of shelf-margin grainstone and a relative absence of marine cement in shelf-interior lithologies allows for extensive development of porosity-enhancing diagenetic processes that are related to small-scale and large-scale relative sea-level fluctuations (e.g., glacio-eustasy). Diagenetic processes that have been mapped and identified by petrographic and geochemical study include marine-related dolomitization, mixing-zone dolomitization, and meteoric leaching. In the field, reservoir development is expressed as: (1) early marine-related dolomitization of shelf-margin oolite and interior lagoonal sediments, which results in formation of intercrystalline and moldic porosity, and thin stratal reservoirs at a scale of meters to tens of meters; and (2) karsting of platform-interior lithologies, which results in formation of vugs, molds, and microporosity, and thin and thick stratal reservoirs at the scale of meters to < 100 meters. The observed changeover from Upper Devonian-Tournaisian meter-scale peritidal carbonate cycles to Visean-Serpukhovian meter-scale carbonate cycles capped by paleosols and carbonate-clast conglomerates is interpreted to reflect a changeover from “greenhouse” to “icehouse” conditions during the Middle to Late Visean. Thus, Visean to Serpukhovian shelf carbonates are characterized by sequential meter-scale layering and early meteoric diagenesis, and Bashkirian reservoirs are more susceptible to diagenetic enhancement due to original aragonite composition. The sequential deposition of Upper Devonian through Lower Permian carbonates results in a thick stack of relatively tight lithologies at platform margins and slopes, and porous lithologies within shelf margins and interiors. Comparison of predicted diagenetic patterns and reservoir geometries based on field studies with observed diagenetic and reservoir attributes of Karachaganak and Tengiz fields suggest that similar diagenetic processes were operative in the North Caspian basin.

Abstract Outcrops of intact and seismic-scale Pennsylvanian (Serpukhovian to Moscovian) carbonate platforms in Asturias (NW Spain) were studied as analogues of the prolific subsurface reservoirs in the Pricaspian Basin (e.g., Tengiz Field). The Asturian platforms, which have been rotated 90° along the dip axis during Late Carboniferous thrusting, are visible on aerial photographs as kilometer–scale cross sections and have dimensions similar to their Pricaspian subsurface equivalents: a thickness between 1.5 and 2.0 km, a slope relief up to 850 m, slope angles up to 32°, and oblique–exponential clinoforms. A comparative study of stratal patterns, lithofacies, and petrophysical properties using aerial photography was initiated to: (1) develop a depositional model, (2) construct a seismic model for comparison with the Pricaspian subsurface, and (3) address the controls on slope declivity. Five general lithofacies–stratal pattern zones were observed: inner and outer platform, upper slope, lower slope, and toe–of–slope to basin. The platform zone has shoaling–upwards cycles with a transgressive interval of coated grainstone with oncoids, followed by normal marine algal boundstone and bioclastic grainstone to packstone and, near the top, restricted lagoonal peloidal packstone to grainstone with calcispheres. These cycles have a thickness between 2.5 and 15 m and can be traced from the platform break into the platform interior for at least 6 km. Crinoid–bivalve grainstone to rudstone intervals and lenticular mud mounds are present in the outer platform, a one– kilometer–wide zone near the platform break. Two distinctly different automicrite margins are recognized in the field: (1) low–angle slopes and ramps, deposited during the nucleation phase of the platform, of nearly pure micritic limestone, and (2) steep (26 to 32°) slopes where automicrite boundstone dominates the uppermost 300 m. Clotted peloidal micrite—automicrite—with sponges and fenestellid bryozoans, and crinoid rudstone intervals, dominate this zone. Below 300 m paleo–water depth, clast–supported lithoclastic breccia dominates the slope. Finally, below 600 to 700 m, argillaceous lime mudstone beds interfinger with grainstone to wackestone intervals of mostly platform– top–derived grains and thick intervals of upper–slope–derived breccia. Five major phases of platform development are recognized: (1) renewed flooding of the preexisting regional Serpukhovian platform, rucleation of a low-angle ramp with microbial mud deposits, aggradation and subsequent formation of a steep microbial cement boundstone margin, followed by nearly horizontal progradation (Bashkirian), (2) continued progradation with several aggradational phases (Bashkirian), (3) development of an extensive flat-topped shallow-water platform near the Bashkirian-Moscovian boundary followed by combined aggradation and progradation, (4) predominantly progradation, followed by (5) aggradation. On the steep upper slopes, over 30°, automicrite formation alternated with deposition of sand and rubble. Automicrite layers slid off and formed breccia tongues at the toe of slope whenever the shear strength of the substrate of loose sediment was exceeded. The steep slope angles were maintained by alternating automicrite growth stages and gravity-driven deposition and consequently inhibited the growth of large mud mounds. Calibration of lithofacies and stratal patterns in a large-scale platform outcrop with their potential seismic expression through synthetic seismic modeling shows great similarity with seismic data acquired in the Pricaspian subsurface. The integration and quantification of size-similar outcrop data is a first step in developing a powerful predictive tool for the exploration of the subsurface of the Pricaspian Basin.

Abstract Lower Permian sediments of the southern Urals (Russia) constitute one of the main carbonate systems with hydrocarbon and gas accumulations. The reservoir facies all along the Pricaspian Basin, the Timan–Pechora belt, and the Russian Platform originated mainly as reef complexes. In this work, reservoir–rock facies have been studied in two outcrops of the Sterlitamak–Ishimbaevo area, the so–called Tratau and Shaktau shikhany. Their complex reef architectures, recognized as pinnacles, display three associated facies: reef–core facies (composed of coral framestones, Archaeolithoporella bindstones, palaeoaplysinid platestones, Tubiphytes bindstones, and bryozoan cementstones), reef– flank facies (bedded floatstones and packstones), and inter–reef and off–reef facies (packstones and grainstones alternating with marlstones). Five depositional phases are envisaged for the architecture of both reef complexes, which were developed from Asselian to late Artinskian times. A detailed sequence stratigraphic framework of both reef complexes, governed by the interplay of regional and local tectonically induced subsidence and sea–level fluctuations, is examined in this work. The Tratau reef complex comprises one complete large–scale sequence, which can be subdivided into transgressive and regressive trends. The top of this reef complex was affected by subaerial exposure in the latest Asselian, and subsequently drowned by local tectonic activity. In contrast, the Shaktau reef complex records four large–scale sequences, in which the top of the first (latest Asselian) and third (latest Sakmarian) are bounded by subaerial surfaces. A widespread regression is recorded from early Asselian to latest Sterlitamakian (Sakmarian) times, which is punctuated by minor transgressions in the Tastubian and Sterlitamakian. In addition, local tectonic activity induced the development of a forced retrogradation in the Shaktau reef complex throughout the Asselian–Sterlitamakian interval. At the end of this episode (Late Artinskian), half grabens were drowned and carbonate deposition was replaced by fine–grained siliciclastics reflecting a widespread transgression. From the middle Carboniferous until the Late Permian, outbuilding and backstepping of the entire Russian platform margin was caused by tectonic pulses, which reflect the migration of the eastern foreland basin.

Abstract The Kozhim carbonate bank, a 380-m-thick Upper Carboniferous-Lower Permian carbonate buildup exposed in the northeastern Komi Republic, Russia, grew on a topographic high near the margin of the Ural Trough. The carbonate bank is composed of intermixed radiaxial cement-rich biohermal facies and associated skeletal wackestones-packstones-grainstones. Traditionally the carbonate buildup has been referred to as a reef, but percentages of biohermal facies are uncertain and organic boundstones are not present, and in this study it is referred to as a carbonate bank. Bank growth was initiated in the Late Kasimovian-Early Gzhelian in response to subsidence of the Ural Trough. In the Gzhelian-Asselian lower part of the bank section, biohermal facies are composed of fenestrate bryozoan- Tubiphytes -radiaxial calcite cementstones. Within the bioherms and associated bioclastic facies, bryozoans, Tubiphytes, echinoderm ossicles, and brachiopods are common to abundant; and ammonoids, gastropods, and smaller foraminifera are sparse. Fusulinids, calcareous algae, and calcisponges occur locally at some horizons but are lacking through most of the section. These periodic occurrences of warmer-water biota probably indicate relative sealevel changes, but no evidence of subaerial exposure was found, and bank deposition appears to have been continuous. In the Late Asselian-Sakmarian upper part of the bank, the fenestrate bryozoan- Tubiphytes -radiaxial cementstones grade into and intermix with Palaeoaplysina bafflestones- cementstones. Palaeoaplysina -phylloid algal bafflestones-cementstones are the dominant facies near the top of the buildup. The Gzhelian-Asselian fenestrate bryozoan -Tubiphytes cementstone bioherms grew in a relatively cool-water deep-ramp to slope setting. The gradual transition in the upper bank from cooler-water facies to a warmer-water Palaeoaplysina -phylloid algal biohermal community suggests upward bank growth into shallower and warmer water. The upper Palaeoaplysina-rich part of the Kozhim bank is commonly cited as an outcrop analog for Upper Carboniferous-Lower Permian hydrocarbon reservoirs in the adjacent Timan-Pechora Basin that contain similar Palaeoaplysina bioherm facies. However, those reservoir facies are generally associated with shallow-water grainstone shoal deposits that often display evidence of repeated periods of subaerial exposure, and they did not undergo pervasive early diagenetic cementation. Palaeoaplysina bioherm complexes and associated flanking grainstone deposits exposed on Spitsbergen and Bjornoya in the Norwegian Barents Sea are probably better outcrop analogs for the Timan- Pechora reservoir sections.

Abstract For decades virtually all of the former USSR was closed to non-Soviet bloc geologists for conducting collaborative geologic studies. This was unfortunate, inasmuch as this immense territory houses much of the earth’s geological history and is estimated to have approximately 22 percent of the world’s known petroleum volumes. Outside of the Russian literature and a limited number of papers and books translated into non-Russian languages, details about the petroleum resources and the evolution of carbonate rock complexes in the former USSR have been infrequent. The Paleozoic carbonate systems of the CIS are scientifically important because they include many of the world stratotype localities. Moreover, Paaleozoic carbonates of the CIS profide a considerable record from which to evaluate the evolution of reef-building organisms through time, variations in paleoclimate, changes in global sea level, paleotectonics, and secular variation in the composition of ancient sea water, and primary mineralogy of marine carbonate precipitates. All of these variables have a major impact on the sedimentation, accumulation, and diagenesis of carbonate rockes, and thus a better understanding of carbonate systems of the CIS will advance our general knowledge of carbonate sedimentology and carbonate reservoirs.

Abstract Pimienta-Tamabra(!) is a giant supercharged petroleum system in the southern Gulf of Mexico with cumulative production and total reserves of 66.3 billion barrels of oil and 103.7 tcf of natural gas, or 83.6 billion barrels of oil equivalent (BOE). The effectiveness of this system results largely from the widespread distribution of good to excellent thermally mature, Upper Jurassic source rock underlying numerous stratigraphic and structural traps that contain excellent carbonate reservoirs. Expulsion of oil and gas as a supercritical fluid from Upper Jurassic source rock occurred when the thickness of overburden rock exceeded 5 km. This burial event started in the Eocene, culminated in the Miocene, and continues to a lesser extent today. The expelled hydrocarbons started migrating laterally and then upward as a gas-saturated 35–40°API oil with less than 1 wt.% sulfur and a gas-to-oil ratio (GOR) of 500–1000 ft 3 /BO. The generation-accumulation efficiency is about 6%.

Abstract T he need to expand the search for energy resources into deeper marine environments has intensified the importance of better understanding the nature and origin of carbonate slope settings and of acquiring a working knowledge of their characteristics. Coarse-grained mass-flow deposits beyond the shelf break in clastic environments are known to form major petroleum reservoirs (Barbat, 1958), and it is likely that clastic deep-water environments will continue to be future exploration targets (Hedberg, 1970; Curran et al, 1971; Gardett, 1971; Nagel and Parker, 1971; Yarborough, 1971; Schlanger and Combs, 1975; Walker, 1978; Wilde et al, 1978). With the concept of plate tectonics, seismic stratigraphy, and advances in seismicreflection technology, a more sophisticated approach to understanding the developments of continental slopes has emerged (Doyle and Pilkey, 1979). Consequently, this understanding has placed more emphasis on the geological history and petroleum potential of slope settings (for example, Hedberg, 1970; Burk and Drake, 1974; Weeks, 1974; Bouma et al, 1976; Thompson, 1976; Wang and McKelvey, 1976; Bloomer, 1977; Schlee et al, 1977; Mattick et al, 1978). Well-documented examples of petroleum reservoirs in carbonate slope and basinal settings are fewer in number than their clastic counterparts. However, it is likely that more deep-water carbonate reservoirs will be discovered as exploration and research continue in this domain (Cook et al, 1972; Cook, 1979a;Cook and Enos, 1977b; Enos, 1977a, b, in press; Scholle, 1977; Flores, 1978; Mullins et al, 1978; Mullins and Neumann, 1979; Cook and Egbert, 1981a; Enos and Moore, this volume).

Abstract The increased need to find new energy resources in deep marine frontier environments has clearly intensified the importance and interest in deep water carbonate settings and how these settings interrelate to adjacent shoal water platform margins. Coarse-grained mass-flow deposits beyond the shelf break in terrigenous clastic environments have been known for many years to form major petroleum reservoirs (Barbat, 1958), and it is likely that similar deep-water clastic facies will continue to be future exploration targets (Hedberg, 1970; Curran et al, 1971; Gardett, 1971; Nagel and Parker, 1971; Schlanger and Combs, 1975; Walker, 1978; Wilde et al, 1978; Howell and Normark, 1982). With the concept of plate tectonics, seismic stratigraphy, advances in seismic-reflection technology and cycles of relative sea level change, a more sophisticated approach to understanding the developments of deeper water environments has emerged (Cook and Enos, 1977a, b; Doyle and Pilkey, 1979; Stanley and Moore, 1983), Consequently, this understanding has placed more emphasis on the geological history and petroleum potential of slope and basin margin settings (for example, Hedberg, 1970; Burk and Drake, 1974; Weeks, 1974; Bouma et al, 1976; Thompson, 1976; Wang and McKelvey, 1976; Bloomer, 1977; Schlee et al, 1977; Mattick et al, 1978; Krueger and North, 1983). Well-documented examples of petroleum reservoirs in carbonate slope and basinal settings are fewer in number than their terrigenous clastic counterparts. However, discoveries of major petroleum accumulations in upper Paleozoic-lower Cenozoic slope facies have stimulated interest in deep water carbonates (Cook et al, 1972; Enos, 1977a, in press; Viniegra-O