The 10 most-cited articles in AAPG Bulletin, 2024

ABSTRACT Organic-rich continental and marine–continental (i.e., transitional) shales are characterized by numerous hydrocarbon production layers having an uneven horizontal distribution, which are challenging to locate and exploit. We examined the effects of karst topography on organic carbon accumulation during the early Permian in the southeastern Ordos Basin, northwestern China, using outcrop and well data. Our study involved geomorphological, sedimentological, petrological, and geochemical methods. We identified a regional unconformity on the Dongdayao Limestone (DDYL) that formed in the early Permian (Asselian; i.e., in the Shanxi Formation) in the study area based on (1) cave, pore, and breccia development in outcrops and drill cores; (2) high Mn–Fe and low Sr contents associated with negative δ18O and normal δ13C values, which are indicative of strong leaching by meteoric waters; and (3) the irregular thickness of the DDYL that is indicative of differential karstification, resulting in the formation of horizontal gullies. The karst topography of the DDYL was identified based…

ABSTRACT The lower Cambrian and lower Silurian shale reservoirs of southern China display significant differences in pore characteristics that may reflect the varying effects of silica diagenesis. The lower Cambrian and lower Silurian mudstone samples from the middle–upper Yangtze platform were analyzed to elucidate silica diagenetic modifications, including their influences on pore evolution. Quartz of the studied mudstone samples includes detrital quartz, overgrowths, siliceous skeletal fragments, microquartz aggregates, silica nanospheres, and matrix-dispersed microquartz. Much of the authigenic silica precipitated in association with diagenetic alteration of the studied deposits appears to have been derived from dissolution of siliceous skeletal fragments and the smectite–illite reaction. A paucity of siliceous organisms populating the early Cambrian ocean gave rise to more complicated silica diagenetic pathways during alteration. Silica precipitation from silica-rich seawater/pore water and silica replacement by carbonate/carbonate-fluorapatite may have supplemented diagenetic…

ABSTRACT The petroleum system concept and related theory are systematically reviewed in this work using published studies. In light of the recent success in exploiting unconventional petroleum resources, the classic petroleum system concept cannot be used to account for the vast unconventional oil and gas accumulations within the petroleum system. The whole petroleum system model is here adopted to investigate both the conventional and unconventional hydrocarbon resources in the Mahu sag, Junggar Basin, China. The whole petroleum system in the study area developed in Permian strata deposited within a large, alkaline lacustrine basin, comprising conventional and unconventional reservoirs that coexist orderly. The Permian Fengcheng Formation in the Junggar Basin is a unique alkaline lacustrine source rock that ranges from low maturity to high maturity, contains abundant organic matter and volcanic materials, and has a high oil-generating potential. Different types of hydrocarbon reservoirs are distributed from the margin to the center of the sag and vary from conventional fan-delta…

ABSTRACT Petroleum migration and accumulation associated with ostracod-bearing layers in shale oil systems were investigated using a large suite of high-resolution geochemical and mineralogical data combined with petrology description and pore characterization of shale core samples from the first member of the Upper Cretaceous Qingshankou Formation in the Qijia-Gulong sag of the Songliao Basin, northeastern China. The first member of the Qingshankou shale, deposited in a moderately deep lake setting, is dominated mainly by massive mud rock and laminated shale with numerous ostracod-enriched interbedded layers. The thin carbonate-rich ostracod-bearing layers dominate the total organic carbon (TOC)–lean facies having TOC values of <2 wt. %. Petroleum yields (volatile free hydrocarbons in programmed pyrolysis [S1]) and solvent extract yields share similar vertical variation trends with TOC content, indicating that TOC content exerts an important control on retained hydrocarbon. However, there exist some upward increasing trends of TOC-normalized petroleum yields (S1) and solvent…

ABSTRACT The adjustment of organic matter (OM) molecular structure due to graphitization should be of great significance to the evolution and preservation of OM pores at an extremely high maturity stage. In this study, the lower Paleozoic Silurian Longmaxi contact metamorphic shales from the north section of the Xuefeng Mountain tectonic zone of the South China block were taken as an example and the molecular structure of kerogen in these postmature Longmaxi shales was analyzed using laser Raman microprobe, transmission electron microscopy, and Fourier transform infrared spectroscopy techniques. The OM pore structures of shale samples with different thermal maturity were compared using CO2/N2 adsorption and field emission scanning electron microscopy analysis. The results indicate that postmature kerogens have entered the transition stage from amorphous carbon to crystallized graphite. The orderliness and crystallinity of carbon atom layers in these postmature samples continuously increase with maturity, accompanied by decreased disordered graphite lattice. The minimum d (Å) value…

ABSTRACT Enhanced hydrogeologic circulations promoted by tectonics are commonly linked to karstic cavity formation in carbonate rocks, providing superb reservoirs for hosting energy resources (i.e., hydrocarbon and geothermal) in sedimentary basins. Predicting such cavern reservoirs in the deep subsurface is difficult mainly due to uncertainties in timing the tectonics and characterizing their associated fluids, which hamper the related hydrocarbon exploration. By combining carbonate U-Pb chronology, geochemistry, and seismic data analyses of fracture and cave-filling carbonates in cavern reservoirs from the Ordovician units of the Tarim Basin, northwestern China, the current study sought new evidence for fluid activities related to tectonics. Crucially, carbonate U-Pb ages confirm that these karstification events were closely related to syn- and/or postmineralization faulting by local tectonics. Geochemistry signatures in the authigenic minerals of fractures further suggest that the episodically developed meteoric water mixed with deep basinal brine. The carbonate dissolution rate…

ABSTRACT Lacustrine organic-rich shales have recently become important petroleum exploration targets. Adequate reservoir characterization is vital for determining the potential for shale oil exploration and development. Fluid–rock interaction and diagenetic mass transfer in organic-rich shales are essential processes during shale oil reservoir formation. Based on detailed petrographic investigation, in situ element and isotope measurement, and organic geochemistry analysis, diagenetic mass transfers and related organic–inorganic interactions were investigated using a suite of organic-rich shales from the Triassic Yanchang Formation of the Ordos Basin. Organic-rich shales consist of silt-sized felsic laminae and organic-rich laminae. Silt-sized felsic laminae are dominated mainly by K-feldspar, whereas illite is the most abundant mineral in organic-rich laminae. Authigenic quartz and euhedral pyrite are the major diagenetic minerals in organic-rich laminae, whereas K-feldspar dissolution occurs extensively in silt-sized felsic laminae. Smectite-to-illite conversion has played a…

ABSTRACT Karst reservoirs in the Ordovician carbonates of Tarim Basin are currently major targets of deep and ultra-deep hydrocarbon exploration and development in China. New drilling and seismic data show that these deep-buried carbonate karst reservoirs are spatially dependent on strike-slip faulting. Based on the geological and geophysical data, this study developed an attempt to determine the impact and spatial distribution of strike-slip faulting on carbonate karst reservoirs in the Tahe oil field. Strike-slip faults in the Taha area can be classified as first order, second order, and third order. Fault damage zones are distributed along the strike-slip faults, and their widths have a positive correlation with fault order. Influenced by strike-slip faults, fault-related fractures, and rock mechanical properties, the development degree and spatial distribution of carbonate karst reservoirs display strong heterogeneity. The horizontal heterogeneity of karst reservoirs is controlled by strike-slip faults with different scales and strikes, and the vertical heterogeneity is combined…

ABSTRACT Produced water (PW) associated with oil and gas production from unconventional reservoirs in the Permian Basin has the potential to constrain future production if not managed effectively. The objective of this work was to report trends in produced and disposed water volumes and to highlight disposal capacity challenges and implications for future development. We compiled production and disposal data from public and commercial databases, focusing on the Midland and Delaware Basins. Results show that annual PW volumes from unconventional wells in the Permian Basin increased by a factor of 30 from 2010 to 2022. Unconventional PW volumes in 2022 totaled approximately 5 billion bbl, with 70% from the Delaware Basin and 30% from the Midland Basin. This increase reflects increasing numbers of horizontal wells drilled (approximately 6000 in 2022), lateral lengths (approximately 2 mi in 2022), and water to oil ratios (averaging 3.3 in 2022 and up to 10 locally). Currently, almost all PW from unconventional production is managed by disposal into strata shallower (75%) or deeper (25%)…

ABSTRACT Fault-controlled reservoirs are critical hydrocarbon-bearing reservoirs in carbonate rocks. The ultradeep Ordovician fault-karst reservoirs in the Tarim Basin are representative carbonate reservoirs whose formation was controlled by tectonics. An integrated analysis of seismic, drilling, logging, and core data and the petrology and geochemistry of the area suggest that the strike-slip fault, lithology, and diagenetic fluids affected the size and formation of the fault-karst reservoirs. The meteoric water dissolution during episodes I–III in the middle Caledonian Orogeny was critical in the formation of the fault-karst reservoirs. In contrast, organic acid, magmatic-hydrothermal fluid, and Mg-rich high-salinity fluid had a negligible effect on the development of these reservoirs. The impact of meteoric water was likely limited in the strike-slip fault zone due to its short exposure time. The releasing bend, grainstone, and packstone were more conducive than the restraining bend, single fault, wackestone, and mudstone to the development of the fault core and fault damage zone…