The 10 most-cited articles in AAPG Bulletin, 2018

ABSTRACT The strike-slip fault systems in the central Tarim Basin, China, afford an exceptional opportunity to document the structural characteristics and evolution process of small displacement intracratonic strike-slip faults using three-dimensional seismic reflection data. These strike-slip faults display subvertical segments at depth and en echelon normal fault zones where relatively shallow. Fault segmentation and flower structures can be commonly observed in plan view and cross-section view, respectively. Consistent with the notion that segment coalescence is the fundamental process for fault evolution, the mean segment length of representative strike-slip faults examined in this study is positively correlated to the measured fault offset. The width of the en echelon normal fault zone is positively correlated with the estimated maximum overburden thickness. The integrated data sets suggest that the evolution of the conjugate fault array followed a sequential evolution process instead of forming simultaneously. The switch in slip direction of the master fault of the conjugate…

ABSTRACT The global Precambrian–Cambrian system includes an important series of hydrocarbon-bearing strata. However, because rocks of this age are typically deeply buried, few petroleum exploration breakthroughs have been made, and the presence of source rocks remains somewhat controversial. Recently, commercial condensate and gas were discovered from the deep (∼6900 m [∼22,600 ft]) Zhongshen 1C (ZS1C) exploratory well drilled in the Tazhong uplift of the Tarim Basin, China, leading to renewed interest in the development of Cambrian source rocks in the basin. On the basis of outcrop reconnaissance and sample testing from around the Tarim Basin, we show that a set of high-quality source rocks were developed within the lower Cambrian Yuertusi Formation (Є1y), at the base of the lower Cambrian. These rocks are black shales and typically have a total organic carbon content between 2% and 6% but extending as high as 17% in selected regions. This marine sequence is 10–15 m (33–49 ft) thick in some outcrops along the margins of the basin. Seismic data indicate that these high-quality…

ABSTRACT Assessing quantitatively the microscopic pore structures of porous rocks, including irregularities of pore shapes and pore size distributions, is becoming one of the most challenging efforts. Nuclear magnetic resonance (NMR) measurements were used to provide insights into the pore geometry (pore size and shape) and pore connectivity of the Chang 7 tight shaly sandstones (in situ permeability <0.1 md) in Ordos basin. The incremental transverse relaxation time (T2) distributions for the 100% brine-saturated samples display unimodal and bimodal behaviors, presenting a geometrical arrangement composed of small to large pore size domains. The NMR parameters such as bulk volume irreducible (BVI) (capillary and clay-bound water), free fluid index (FFI) (movable water), the value of T2 separating the BVI from FFI, the amplitude weighted mean on a logarithmic scale (T2gm), and the value of T2 that shows the highest frequency on the T2 spectrum (T2peak) for each sample were determined. Then the fractal theory was adopted to quantitatively express the complexity and heterogeneity of…

ABSTRACT Solid bitumen is a common organic component of thermally mature shales and typically is identified by embayment against euhedral mineral terminations and by groundmass textures. However, because these textures are not always present, solid bitumen can be easily misidentified as vitrinite. Hydrous-pyrolysis experiments (72 hr, 300°C–360°C) on shale and coal samples show that solid-bitumen reflectance (BRo) in shales is less responsive to thermal stress than vitrinite reflectance (Ro) in coal. This effect is most pronounced at lower experimental temperatures (300°C–320°C), whereas reflectance changes are more similar at higher temperatures (340°C–360°C). Neither a “vitrinite-like” maceral nor “suppressed vitrinite” was identified or measured in our sample set; instead, the experiments show that solid bitumen matures slower than vitrinite. The data may explain some reports of “Ro suppression,” particularly at lower thermal maturity (Ro ≤ 1.0%), as a simple case of solid bitumen being mistaken for vitrinite. Further, the experimental results confirm previous empirical…

ABSTRACT The pore structure of shale has a significant effect on hydrocarbon migration and the long-term gas supply of shale gas wells. The present study investigates the spontaneous imbibition characteristics to evaluate the pore connectivity and wettability of marine Longmaxi shale samples from the southeastern Chongqing area and continental Yanchang shale samples from the Ordos Basin. The pore-size distribution obtained from N2 adsorption and mercury intrusion porosimetry, field emission–scanning electron microscopy, and focused ion beam–scanning electron microscopy photos are used to interpret the imbibition behaviors. Our results show that the difference in dominant pore type between marine and continental samples, which is dominated by thermal maturity, controls on their imbibition behaviors as well as their wettability. Organic matter (OM) pores within Yanchang samples are poorly developed because of their low thermal maturity, and a large amount of water-wet inorganic pores are preserved in these samples because of relatively weak compaction. Oil-wet OM pores are well…

ABSTRACT The equivalent pore aspect ratio provides a tool to detect pore types by combining compressional and shear wave velocities, porosity, bulk density, and mineralogical composition of carbonate rocks. The integration of laboratory measurements, well logs, and petrographic analysis of carbonate rocks from various depositional and diagenetic settings (Lower Cretaceous presalt nonmarine carbonates from offshore Brazil; Lower Cretaceous shallow-water platform carbonates from southeast France; and Upper Cretaceous deep-water carbonate from the Gargano Peninsula, Italy) allowed the acoustic signature of a wide range of pore types to be quantified. The equivalent pore aspect ratios are shown to be independent of pore volume and mineralogy. They can, therefore, be regarded as relevant parameters for discriminating selected pore network architectures in carbonate reservoirs. Interparticle and intercrystalline microporosity and macroporosity may include a variety of distinct pore network structures and a wide range of acoustic signatures depending on particle shape, nature of contacts,…

ABSTRACT The Junggar Basin is one of the major petroliferous basins in Northwest China, where deeper hydrocarbon exploration has become an inevitable trend for resource development. In this paper, the structural features and tectonic evolution of the Junggar Basin are discussed based on seismic profiles, drilling data, isochore maps, and additional balanced cross sections. Our results suggest that the basin has undergone five extension-to-compression stages of tectonic evolution since the Permian. The Junggar Basin initially developed from several isolated continental rifts in the early Permian. At the beginning of the middle Permian, the basin developed into a more integrated terrigenous lake basin with the deposition of fine-grained, organic-rich lacustrine sediments; the basin inverted during the late Permian compressional uplifting. Then the sedimentary basin was superimposed and reworked by episodic Mesozoic to Cenozoic tectonic events, giving rise to the uplift and erosion at the basin margins while folding or tilting within the basin. At present, the basin presents a…

ABSTRACT Image logs provide critical data for characterizing fractured reservoirs. However, the nature of such logs, coupled with borehole-modified stresses, creates challenges that may lead to erroneous fracture interpretations. We developed a methodology to integrate image logs, core, and geomechanical models to obtain high-confidence, fracture data sets derived from image logs. The methodology starts with a detailed comparison of overlapping image logs and core, first to properly shift them into alignment (core-to-log shift) and second to determine which features interpreted as fractures on image logs (picks) can be validated as fractures in core. Then we derive guidelines for interpreting image logs without core control to reduce the likelihood of false fracture picks. Geomechanical modeling is used to assist with quality control and interpretation of image logs where drilling-induced tensile fractures occur. We use a fractured carbonate reservoir to demonstrate the application of this methodology. Comparison of image log fracture picks with core led to an average rejection rate…

ABSTRACT The giant natural gas accumulation recently discovered within the (Neoproterozoic) Dengying Formation in the Sichuan Basin of southwestern China represents a new and important supplement to the ancient Neoproterozoic–Cambrian petroleum systems worldwide. However, the source of this gas is controversial because there are five possibilities based on geology, from bottom to top including the Lower Sinian Doushantuo mudstone, the Upper Sinian Dengying algal dolomite, the mudstone of the third member of the Dengying Formation, the Lower Cambrian Maidiping argillaceous dolomite, and the Lower Cambrian Qiongzhusi shale. To improve the understanding of this issue, 33 samples from the five possible source rock units in the study area were investigated for their character and maturity via petrology, total organic carbon (TOC), pyrobitumen reflectance, kerogen carbon isotope, and biomarker analyses. Based on the results, we address their hydrocarbon generation capabilities and contributions to the giant gas accumulation. Results show that present-day TOC values of the samples range…

ABSTRACT Sediment supply to the ocean influences basin-margin growth and reflects upstream landscape evolution, including patterns of sediment routing, denudation, and tectono-climatic perturbations in source areas. Constraining sediment supply is useful for inputs to stratigraphic forward models and for predictions of reservoir presence and quality. Because of the importance of sediment supply, geoscientists have developed various methods to estimate it. Here, we apply Monte Carlo simulation (MCS) to the BQART model that is used to describe an empirical relationship between river catchment paleogeography, climate, and sediment load. We calculate a range of sediment supply from North American source areas to the Gulf of Mexico that suggests an overall decrease in median sediment supply from the late Paleocene to the early Eocene from 404–514 to 144–204 million tons per year, depending on the published paleogeographic model that we used to guide our selection of input variables. Comparison of these estimates with downstream sediment records shows that the subsurface depositional…