The 10 most-cited articles in AAPG Bulletin, 2021

ABSTRACT Many of those assessing geochemical data have taken a “cookbook” approach to interpretation. Such an approach generally does not take into consideration the limitations of the different data sets. Among these data sets are those associated with thermal maturity, which guides the interpretation of the extent of generation and hydrocarbon phase. The temperature of peak pyrolysis generation (Tmax) is commonly used to map thermal maturity directly or through a conversion to vitrinite reflectance equivalent values. Such efforts do not take into consideration analytical uncertainties or natural variability within a stratigraphic unit at any given locality. Furthermore, when presented as vitrinite reflectance equivalents, the associated error in the conversion is not considered, yet the values are presented as “absolutes.” When examining vitrinite reflectance, there are also several issues that should be considered. Individual mean values are commonly considered out of context. All thermal maturity indicators, including vitrinite reflectance, need to be placed into a geologic…

ABSTRACT Over the last few decades, significant advances in acquisition of new three-dimensional seismic data sets have revealed the existence of a spectacular strike-slip fault system consisting of multiple fault networks developed across the northern Tarim Basin. These strike-slip fault networks are of significant importance to petroleum exploration because of their dual roles as conduits for hydrocarbon migration and fault-controlled paleokarst reservoirs. In this study, two representative strike-slip fault networks are selected for detailed analysis to understand their control on hydrocarbon accumulation and distribution. In the “X-shaped” TP39-TP29 fault network developed in the Tabei uplift, high-production wells are distributed mainly along the first-order north-northeast–oriented fault TP39, particularly in the pull-apart structures. This is attributed to the reactivation of fault TP39 in the late Hercynian and Himalayan orogenies when oil accumulation mainly occurred, whereas the north-northwest–striking faults remained inactive. In the TP39-TP29 fault network, areas of…

ABSTRACT Distribution of volcanic reservoirs in Xujiaweizi half graben is controlled by both faults and sealing layers where their integrity could have been compromised. This paper documents the distribution of sealing units of the Denglouku and Yingcheng Formations based on seismic and well-log data to delineate all of the reservoirs with proper cap rocks within the volcanic Yincheng Formation via the relationship between the fault system and reservoir rocks. Based on the regional sealing effect of Denglouku Formation, two hydrocarbon traps are identified: the lower primary and upper secondary gas reservoirs where the formation with a mudstone percentage of more than 50% within the second member seals the primary reservoirs. The upper secondary trap is controlled by faults that have been activated during the structural reversal phase, causing the regional sealing layers in the Denglouku Formation to get displaced with a cap rock juxtaposition thickness of less than 35 m. This has created a series of fault-seal, dual-control reservoirs. The local seals within the Yingcheng Formation…

ABSTRACT The purpose of this research was to examine paleoenvironments, hydrothermal activity, and seawater restriction of the lower Cambrian Niutitang Formation shale gas reservoir in the eastern Xuefeng uplift and to determine factors affecting organic matter (OM) enrichment. In the studied borehole Xiangan 1 well in western Hunan Province, the Niutitang Formation can be subdivided into the Niu1, Niu2, and Niu3 Members based on geological and geochemical features, including trace element enrichment, lithology, and fossil content. Total organic carbon values of the Niutitang Formation are variable, averaging 1.5 wt. % in the Niu1 Member, 12.7 wt. % in the Niu2 Member, and 5.1 wt. % in the Niu3 Member. Paleoclimatic changes were responsible for changes in biota, which impacted patterns of OM enrichment. Climate proxy (chemical index of alteration) and productivity proxies (biogenic Ba, Cu/Al, and Ni/Al) consistently indicate higher paleoproductivity in the Niu2 Member. The Niu1 and Niu2 Members may be affected by hydrothermal events, whereas hydrothermal activity was absent during…

ABSTRACT Cores, thin sections, and cathodoluminescence analysis were integrated to document the occurrence and petrology of dolomites, and their pore systems in the Cambrian of Tarim Basin, China. Depositional facies, pore types, and dolomitization processes of various dolostone reservoir types are determined. Six types of dolomite are recognized, including microbial dolomite, dolomicrite, fabric-retentive dolomite, fabric-obliterative dolomite, fine to medium crystalline dolomite cement, and saddle dolomite cement. Pore systems are dominantly vugs, anhydrite dissolution pores, intercrystalline pores, intercrystalline dissolution pores, fabric dissolution pores, and microfractures. Four porous dolostone reservoirs include sabkha dolostone, seepage-reflux dolostone, burial dolostone, and hydrothermal dolostone. Fractures are an important factor in enhancing reservoir quality in dolostone reservoirs. Conventional wire-line logs and image logs are calibrated with cores and related thin sections. Sabkha dolostone reservoirs are characterized by dark and bright spots on the image logs.…

Abstract Typical meter-scale lithofacies cycles from the Wolfcamp A in the Delaware and Midland Basins comprise basal carbonate facies overlain by calcareous or siliceous mudrocks. Siliceous mudstones are the most organic-rich facies with high total organic carbon (TOC > 3 wt. %), whereas thin carbonate beds have the lowest organic matter (OM) content among the lithofacies present (TOC < 1 wt. %). To determine if the thin carbonate-rich beds serve as either reservoir storage for migrated oil or act as a seal, three core intervals of meter-scale lithofacies cycles in the Wolfcamp A were selected for sampling and were analyzed using x-ray diffraction analysis, TOC, programmed pyrolysis analysis, and residual gas analysis from rock crushing. Oil saturation index (OSI) (the amount of free oil normalized by TOC; OSI = S1 × 100/TOC) is used as an indicator of oil enrichment or depletion in the reservoir, where S1 is volatile oil in programmed pyrolysis (temperature = 300°C). Both TOC-lean carbonate and TOC-rich mudstone lithofacies have high OSI in these meter-scale cycles (average OSI is…

ABSTRACT Recent production of light sweet oil has prompted reevaluation of Devonian petroleum systems in the central Appalachian Basin. Upper Devonian Ohio Shale (lower Huron Member) and Middle Devonian Marcellus Shale organic-rich source rocks from eastern Ohio and nearby areas were examined using organic petrography and geochemical analysis of solvent extracts to test ideas related to organic matter sources, oil–source rock correlation, thermal maturity, and distances of petroleum migration. The data from these analyses indicate organic matter in the Ohio and Marcellus Shales primarily was derived from marine algae and its degradation products, including bacterial biomass. Absence of odd-over-even n-alkane distributions (n-C13 to n-C21 range) in gas chromatograms and low gammacerane index values in Devonian source rocks are similar to those of Devonian-reservoired oils in eastern Ohio, suggesting an oil–source rock correlation. Lower Paleozoic oils from eastern Ohio, in contrast, are characterized by the presence of odd-over-even n-alkane distributions (n-C13 to n-C21 range) and…

ABSTRACT The northeastern Sichuan Basin thrust belt located in southwestern China, is a large-scale intracontinental thrust system with multiple detachments represented by a series of subparallel chevron anticlines. We conduct a comprehensive study of the geometry and kinematics of the thrust belt and its implications for the petroleum system based on seismic-reflection profile interpretation, field investigation, analysis of wells, and geochronology. Two major detachments occur within the allochthonous succession: the (1) gypsum-bearing lower to middle Cambrian Longwangmiao and Gaotai Formations and (2) Lower Triassic Jialingjiang Formation. The dark gray shales in the lower Silurian Longmaxi Formation are a favorable source rock that may also act as a third detachment in this study area. The stratigraphic succession is divided by three detachments into three structural intervals: (1) lower Cambrian–Silurian structural interval, (2) middle Silurian–Triassic structural interval, and (3) upper Triassic–Jurassic structural interval. The lower structural interval may be a good…

ABSTRACT The Paleozoic sedimentary section of the Arabian plate recorded the effects of numerous fluctuations in sea level and gentle epeirogenic movements. Eustatic changes in sea level over the Arabian plate resulted in transgressions and regressions and created many local and regional unconformity surfaces, identified by missing section and nondeposition. The distant deformation of the Caledonian tectonic orogeny and mountain building of the North Atlantic region, and the later Hercynian tectonic event, are partially responsible for eustatic sea-level changes. Additionally, detailed evidence of Paleozoic glacial erosion and glacial eustasy in the regressive sedimentary section over uplifted paleohighs comes from deep wells penetrating local exotic tectonic blocks. The resulting surfaces mark the sea-level falls associated with the glacial events in the Ordovician, Late Devonian, and Carboniferous–early Permian sediments. The depositional settings of the Cambrian–upper Permian rocks include deltaic and glacial settings and are expressed as alluvial–fluvial fans that grade into…

ABSTRACT The West Texas (Permian) Super Basin is the prototype super basin. The basin has produced 28.9 billion bbl of oil and 203 TCF of gas (63 billion BOE, 1920–2019). The US Geological Survey and Bureau of Economic Geology estimate this super basin has remaining reserves of 120–137 billion BOE, twice the volume produced during the first 100 yr of hydrocarbon production. During the past decade, the West Texas Super Basin has been the driver of production growth in the United States and has decades of remaining economic production and reserve growth. The West Texas Super Basin is a complex Paleozoic basin built on a varied Proterozoic crust. After Cambrian rifting, regional subsidence began in the Middle Ordovician and continued into the Devonian, forming the Tobosa Basin. The early Paleozoic Tobosa Basin subsidence terminated during Mississippian epeirogenic uplift. A later stage of subsidence began in the Late Mississippian accompanied by large-scale faulting and moderate folding. This tectonic and structural development was controlled by basement terrains, earlier tectonic, and…