The 10 most-cited articles in AAPG Bulletin, 2019

ABSTRACT The lacustrine shale of the Upper Cretaceous Qingshankou Formation is the principal prospective unconventional target lithology, acting as source, reservoir, and seal. Lithofacies and associated storage capacity are two significant factors in shale oil prospectivity. This paper describes an investigation of the lower Qingshankou Formation lacustrine shale based on detailed description and analysis of cores, shale lithofacies characteristics, depositional setting, and stacking patterns. Seven lithofacies are recognized based on organic matter content, sedimentary structure, and mineralogy, all exhibiting rapid vertical and lateral changes controlled by the depositional setting and basin evolution. An overall trend from shallow-water to deep-water depositional environments is interpreted from the characteristics of the infilling sequences, characterized by increasing total organic carbon (TOC) and total clay content and decreasing layer thickness (i.e., from bedded to laminated then to massive sedimentary structures). Periods of deposition during shallowing cycles show a…

ABSTRACT The Zhang 22 well was drilled in the Ordos Basin, penetrating the Chang 7 Member of the Triassic Yanchang Formation, which, cumulatively, has more than 80 m (>260 ft) of black organic-rich shale of oil window maturity. Using 76 samples collected every 1 m (3 ft) from the well, the effects of stratigraphic fractionation and petroleum expulsion within five intervals of the Chang 7 shale were qualitatively and quantitatively documented. The organic-rich intervals 1, 2, and 5, having an average total organic carbon (TOC) of 6.79 wt. % and pyrolyzable hydrocarbon potential of 9.40 mg/g rock (i.e., the amount of hydrocarbons generated by pyrolysis between 300°C and 650°C [S2]), are defined as “generative units” in the Chang 7 shale system, compared to the “in-source reservoirs” or “sweet spots” (i.e., the third and fourth intervals), which contain a lower average TOC of 4.19 wt. % and an average S2 value of 7.17 mg/g rock, but the highest amount of free oil (average total oil of 7.35 mg/g rock). Geochemical and molecular compositions display distinctive differences between…

ABSTRACT Giant petroleum accumulations worldwide with burial depths more than 7000 m (>23,000 ft) occur mostly in Mesozoic and Cenozoic reservoirs and yield predominantly natural gas. Recently, however, a giant oil accumulation with reservoir depths between 7000 m (23,000 ft) and 8000 m (26,000 ft) was discovered in the lower Paleozoic section in the southern part of the Halahatang region in the Tarim Basin, China. Petroleum sourced from lower Paleozoic rocks is contained in Ordovician karst fracture-cave reservoirs and sealed by Middle–Upper Ordovician limestones and mudstones. The newly discovered superdeep accumulation is among the deepest black single-phase oil accumulations worldwide and opens up new avenues for petroleum exploration in deep-marine carbonate reservoirs. Reservoir pressures are between 75 MPa (10,878 psi) and 85 MPa (12,328 psi), with pressure coefficients between 1.2 and 1.7 and temperatures ranging between 140°C (284°F) and 172°C (342°F). Charging and accumulation of petroleum occurred during the late Hercynian orogeny, followed by subsequent gradual deep…

ABSTRACT The influence of thermal maturity on porosity in shale samples from the Upper Devonian Duvernay Formation is examined. The samples span a maturity range from immature to the wet gas window. Porosity decreases from immature to the oil window, primarily because of compaction. Relatively high porosity of wet gas window samples is ascribed to formation of secondary organic pores, feldspar dissolution pores, and primary pore preservation by the quartz framework. The final decline in the porosity of the dry gas window samples is explained by greater compaction, the disappearance of secondary organic pores, and feldspar dissolution pores. Porosity correlates positively to quartz content and negatively to carbonate content; no relationship was evident between porosity and clay or total organic carbon content. No obvious correlations exist between rock composition and permeability except that SiO2 content shows a weakly positive correlation to permeability. Permeability is highest in immature samples, which have the greatest pore and pore-throat sizes. Nitrogen adsorption and…

ABSTRACT Fractal analysis was performed on carbonate core plug samples from the Ordovician Majiagou carbonate reservoirs in the Ordos Basin using mercury intrusion capillary pressure (MICP), nuclear magnetic resonance (NMR), and the x-ray computed tomography (CT) measurements to improve our understanding of the pore structure characteristics. The relationships between pore structure parameters and the fractal dimensions were investigated. The pore systems are dominated by secondary intercrystalline pores and enlarged dissolution pores as well as microfractures. The fractal curves from MICP analysis break into two segments at the Swanson’s parameter. The small pore-throat systems can be described by the fractal theory, whereas pores connected by relatively large throats (greater than the pore-throat radius at the Pittman’s hyperbola’s apex) are not cylindrical in shape, cannot be described by a capillary tube model, and tend to have apparent fractal dimensions larger than 3.0. The fact that the entirety of the capillary curve cannot be fit by a single fractal dimension implies that…

ABSTRACT The nanometer-scale architecture of organic matter (OM) and associated pores in highly mature gas shales from the lower Silurian Longmaxi Formation in the upper Yangtze platform of south China were investigated using field emission scanning electron microscopy (SEM), focused ion beam SEM, and low-pressure gas (N2 and CO2) adsorption bulk pore characterization. The Longmaxi shale comprises fine-grained siliciclastic rocks deposited in a marine shelf environment, which was dominated by quartz and clay minerals. Four porous OM types were found in the Longmaxi shale on the basis of the chemical composition and spatial occurrence of OM, including (1) isolated original OM particles, (2) OM–clay mineral complexes, (3) OM–heavy mineral complexes, and (4) secondary migrated bituminous OM. The pores in the particulate OM are not homogeneously distributed, and the processes leading to different pores depend on the specific OM type. The nature of OM-hosted pores is a result of several factors, such as primary porous kerogen, mechanical compaction, organic–inorganic interactions,…

ABSTRACT An integrated analysis of the petrographic characteristics and types and distribution of diagenetic alteration in the upper Paleozoic Benxi–Taiyuan, Shanxi, and Xiashihezi Formations provides insights into the controlling factors on variations in porosity and permeability in tight sandstones (85% of the sandstone samples display porosity values <10% and 90% of the samples exhibit permeability <1 md). Diagenetic alteration includes mesogenetic compaction, cementation by dolomite, ankerite, and quartz, dissolution of feldspar, and illitization of smectite. Eodiagenesis includes compaction, development of smectite, cementation by pore-filling quartz and disordered kaolinite, and precipitation of calcite and Fe-calcite. Chlorite and quartz preserve primary pores against damage, whereas kaolinite, illite–smectite (I/S) mixed layer, and illite significantly diminish reservoir quality via permeability reduction. Chlorite and I/S content decrease abruptly as depth increases, whereas the kaolinite content remains elevated at depth because of the complete destruction of K-feldspar.…

ABSTRACT The Fuling gas field in Sichuan Basin, China, has produced greater than 1.5 × 1010 m3 (0.53 tcf) of natural gas from overmature Upper Ordovician Wufeng and lower Silurian Longmaxi shales. To systemically investigate the characteristics of wettability and connectivity and to understand the underlying causes of production behavior, we study five samples of Wufeng and Longmaxi shales with different total organic carbon contents and mineral compositions. Complementary approaches include mercury intrusion capillary pressure (MICP), contact angle measurement, spontaneous imbibition and saturated diffusion, and tracer (both nonsorbing and sorbing) migration mapped via laser ablation inductively coupled plasma mass spectrometry. According to measured contact angles and imbibition tests conducted on aqueous (deionized water and brine) and oleic (n-decane) phases, Wufeng and Longmaxi shales are strongly oil wet and moderately strong water wet. The lower boundary of estimated permeability obtained from n-decane imbibition can reach 137 nd, which is higher than the geometric mean…

ABSTRACT Natural fractures provide the main path for fluid flow in tight oil reservoirs and can control the flow direction in the subsurface. Tight sandstones commonly have intense mechanical anisotropy, which means that fracture development in such tight formations may vary widely with respect to fracture orientations. However, the prediction of the degree of fracture development for each orientation is challenging. Focusing on the tight sandstones of the Chang 4 and 5 Member in the Jiyuan oil field, Ordos Basin, China, a new approach was presented for better prediction of the tectonic fracture occurrence in different directions based on fracture characterization, controlling factor, and formation mechanism analysis. First, fracture types, characteristics, formation time, and controlling factors were determined using data from outcrops, cores, and image logs. Then, triaxial tests were conducted to measure the mechanical parameters of rock samples in different directions that assessed the mechanical anisotropy of the formation and its impact on the development of fracture networks…

ABSTRACT The Isthmian salt basin in the southern Gulf of Mexico can be divided into the Yucatán and Campeche subbasins, separated by a base-salt high near the nose of the Yucatán platform. Despite their proximity, these two subbasins experienced radically different histories in the period immediately following salt deposition. Portions of the Yucatán subbasin are characterized by large-scale (locally as much as 60 km [37 mi]) downdip translation of salt and suprasalt sediments during the Late Jurassic. This translation produced a major detached extensional province at the updip end of the basin, which is not compensated by observed shortening downdip. We interpret this history to be a result of unconfined seaward flow of salt and its cover during basin opening, a process mirrored on the conjugate Florida margin. The Campeche subbasin, in contrast, shows no evidence of significant Late Jurassic translation detached on salt. No large-scale extensional or contractional provinces of Mesozoic age are evident, although some minor translation did occur. We suggest that salt in the Campeche…