The 10 most-cited articles in AAPG Bulletin, 2020

ABSTRACT The Ordovician–Silurian organic-rich shales in the Wufeng and Longmaxi Formations are widespread in southern China and are the most important shale gas producers in China. The shales display varying total organic carbon content ranging from 0.2% to 6% and are composed of black siliceous mudstone, shelly limestone, siliceous clay-rich mudstone, gray silty–shaly interlaminated mudstone, gray argillaceous mudstone, and calcareous or dolomitic mudstone. Detailed stratigraphic correlation of these formations was achieved, and a high-resolution sequence stratigraphy was constructed based on the integration of well-log data, mineralogical, geochemical, and petrophysical analyses. The Wufeng Formation and the Guanyinqiao Bed were interpreted to be a condensed section with four cycles of sea-level fluctuation. The Longmaxi Formation consists of three members. The Long-1 Member was interpreted to be a third-order depositional sequence that can be subdivided into a transgressive systems tract, an early highstand systems tract, and a late highstand systems tract. Within the constructed…

ABSTRACT The University of Texas Hydrate Pressure Coring Expedition (UT-GOM2-1) recovered cores at near in situ formation pressures from a gas hydrate reservoir composed of sandy silt and clayey silt beds in Green Canyon Block 955 in the deep-water Gulf of Mexico. The expedition results are synthesized and linked to other detailed analyses presented in this volume. Millimeter- to meter-scale beds of sandy silt and clayey silt are interbedded on the levee of a turbidite channel. The hydrate saturation (the volume fraction of the pore space occupied by hydrate) in the sandy silts ranges from 79% to 93%, and there is little to no hydrate in the clayey silt. Gas from the hydrates is composed of nearly pure methane (99.99%) with less than 400 ppm of ethane or heavier hydrocarbons. The δ13C values from the methane are depleted (−60‰ to −65‰ Vienna Peedee belemnite), and it is interpreted that the gases were largely generated by primary microbial methanogenesis but that low concentrations of propane or heavier hydrocarbons record at least trace thermogenic components. The in situ…

ABSTRACT Pores within organic matter (OM) are strongly linked to hydrocarbon generation and primary migration in fine-grained source rocks and are very important for evaluating hydrocarbon storage and flow in shale reservoirs. Thus, it is critical to clarify the features of OM-hosted pores and their evolution in organic-rich mud rocks. The OM in mud rocks, including kerogen and bitumen, is deformed when the original equilibrium stress conditions are altered. The OM deformation at the nano- or microscale has rarely been discussed because of the lack of unequivocal evidence. This research begins with examining the subsurface evolution of kerogen and bitumen, with emphasis on various features of pores hosted by bitumen. Evidence of OM deformation is documented in scanning electron microscope images of seven overmature samples from the lower Silurian Longmaxi Shale, Sichuan Basin. To aid in the understanding and analysis, OM deformation is classified into three types: type I deformation induced by one additional force, type II deformation induced by two additional forces acting on two…

ABSTRACT Existing definitions for passive and active salt diapirism are somewhat overlapping and ambiguous. These terms are also equated to downbuilding and upbuilding, respectively, which are problematic concepts as originally conceived and even as subsequently modified. This results in conflicting usage and lack of consensus in the literature, creating confusion and decreasing the utility of the terms. For example, a diapir with a thin roof is defined as active by some but passive by others. In this short contribution, we first review historical definitions and then focus on several inherent problematic aspects including (1) the applicability of the concepts of downbuilding and upbuilding, (2) using the absence or presence and thickness of a diapir roof as criteria to classify diapirs as active or passive, and (3) the related concept of halokinetic sequences. We offer three suggestions in the hope of improving understanding and communication. First, we suggest that the terms downbuilding and upbuilding should be abandoned. Second, we argue that passive diapirism should be used…

ABSTRACT Dolomite reservoirs formed by dissolution are significant for hydrocarbon exploration, but their origins are poorly understood because of their complexity. This study investigated the lower Permian Qixia Formation of the Sichuan Basin (northern Longmenshan Mountains), southwestern China, which has recently been the location of giant natural gas discoveries. Petrological and geochemical analyses were undertaken, including trace elements and C, O, and Sr isotopes. The results indicate that the reservoirs were formed by differential eogenetic karstification and hydrothermal dissolution of grain dolomites. Core and thin-section observations identified three types of dolomite based on grain size: fine- to medium-crystalline dolomite (Dol-1), medium- to coarse-crystalline dolomite (Dol-2), and coarse- to megacrystalline saddle dolomite (Dol-3). The Dol-2 types can be further divided into three subtypes according to the grain structure and degree of pore development: euhedral medium- to coarse-crystalline dolomite with pores (Dol-2a), porous subhedral–anhedral medium- to…

ABSTRACT Super basins are sedimentary basins with more than 5 billion BOE cumulative production and at least 5 billion BOE remaining recoverable resources. Super basins are characterized by two or more source rocks, stacked reservoirs, mature infrastructure, established service sectors, large amounts of data, and access to markets. Super basins commonly have multiple operators that form a community whose competitive atmosphere helps drive efficiency and performance. In an effort to identify sources for future oil and gas supplies, studies of the geology, hydrocarbon richness, and above-ground factors for 400 global basins led to the super basin concept. Super basins have continuous organic-rich mud rocks and other tight reservoirs that heretofore were not considered commercial targets. Based on analogy with proven resource plays in North American super basins, it is now possible to unlock substantial hydrocarbon potential in these tight rocks by employing horizontal drilling and multistage fracturing technologies. Super basins exist both onshore and offshore. The largest super…

ABSTRACT Thermal cracking of organic matter (OM) can enhance gas-shale porosity. However, thermally matured shales are generally characterized by low porosity with high OM content in tectonic exhumation settings. The Wufeng–Longmaxi shale in the Fuling gas field, China, was investigated via bulk analysis and scanning electron microscopy to reveal the influence of tectonic exhumation on the porosity of OM-rich shale. Thermal maturity and organic and mineralogical compositions are regionally homogeneous in the Fuling gas field. Shale porosity is positively correlated with total organic carbon (TOC) and clay mineral content in the gentle and open anticlines, respectively. The OM-hosted pores were preserved with spherical morphologies because of overpressure-associated overburden stress unloading in the gentle anticlines, whereas OM-hosted pores in the open anticlines are characterized by high aspect ratios and are regularly aligned perpendicular to lateral compression stress. A simple simulation confirmed that tectonic folding and compaction during exhumation caused the loss of…

ABSTRACT Grain assemblages in the organic-rich Cline shale in the Midland Basin are dominated by components of extrabasinal derivation (11.4 to 98.5 vol. %; average volume: 82.6%). Major extrabasinal components include K-rich clay minerals, detrital quartz, albite, K-feldspar, micas, and lithic fragments. Intrabasinal components include mainly biosiliceous allochems (sponge spicules and radiolarians), agglutinated foraminifera, Ca-phosphate peloids, clay-rich peloids, organomineralic aggregates, intraclasts, and other biocalcareous allochems. Authigenic minerals are most evident as grain replacements, euhedral ankerite, Ca-phosphate cement, and precipitates in large pores. A strongly localized spatial distribution of diagenetic products at micron to centimeter scales is observed in most siliciclastic samples, except in biosiliceous allochem-rich ones in which abundant intergranular pore-filling clay-size microquartz cement is observed. Compaction is evident in the Cline shale because of low porosity and generally low cement volumes. Neither textural variation nor bulk mineral…

ABSTRACT We present results from 30 quantitative degassing experiments of pressure core sections collected during The University of Texas-Gulf of Mexico 2-1 (UT-GOM2-1) Hydrate Pressure Coring Expedition at Green Canyon Block 955 in the deep-water Gulf of Mexico as part of The University of Texas at Austin–US Department of Energy Deepwater Methane Hydrate Characterization and Scientific Assessment. The hydrate saturation (Sh), the volume fraction of the pore space occupied by hydrate, is 79% to 93% within sandy silt beds (centimeters to meters in thickness) between 413 and 442 m below seafloor in 2032 m water depth. Sandy silt intervals are characterized by high compressional wave velocity (Vp) (2515–3012 m s−1) and are interbedded with clayey silt sections that have lower Sh (2%–35%) and lower Vp (1684–2023 m s−1). Clayey silt intervals are composed of thin laminae of silts with high Sh within clay-rich intervals containing little to no hydrate. Degassing of single-lithofacies sections reveals higher-resolution variation in Sh than is possible to observe in well logs; however, the…

ABSTRACT We interpret the sedimentologic evolution of a deep-water channel-levee deposit in Green Canyon Block 955 (deep-water Gulf of Mexico) by analyzing hydrate-bearing pressure cores and nonpressure cores collected during The University of Texas-Gulf of Mexico 2-1 (UT-GOM2-1) Hydrate Pressure Coring Expedition that preserve remarkable sedimentary structures. The levee is composed of alternating beds of sandy silt and clayey silt that range from millimeters to meters in thickness. We interpret that each couplet of sandy silt and clayey silt records a single turbidity current flow in which the upper part of the flow overtops the levee and is deposited along its flank. The sandy silt is coarser, its beds are thicker, and the fraction of sandy silt to clayey silt (net-to-gross) is greater near the base of the levee. We interpret that as the levee grew, the channel depth increased and a smaller fraction of the flow overtopped the levee. An increase in net-to-gross, both at the base and near the top of the cored section, may record an increase in the size of turbidity current flows or…