Unconventional Petroleum Sedimentology:A Key to Understanding Unconventional Hydrocarbon Accumulation

The commercial exploitation of unconventional petroleum resources(e.g.,shale oil/gas and tight oil/gas)has drastically changed the global energy structure within the past two decades.Sweet-spot intervals(areas),the most prolific unconventional hydrocarbon resources,generally consist of extraordinarily high organic matter(EHOM)deposits or closely associated sandstones/carbonate rocks.The formation of sweet-spot intervals(areas)is fundamentally controlled by their depositional and subsequent diagenetic settings,which result from the coupled sedimentation of global or regional geological events,such as tectonic activity,sea level(lake level)fluctuations,climate change,bottom water anoxia,volcanic activity,biotic mass extinction or radiation,and gravity flows during a certain geological period.Black shales with EHOM content and their associated high-quality reservoir rocks deposited by the coupling of major geological events provide not only a prerequisite for massive hydrocarbon generation but also abundant hydrocarbon storage space.The Ordovician-Silurian Wufeng-Longmaxi shale of the Sichuan Basin,Devonian Marcellus shale of the Appalachian Basin,Devonian-Carboniferous Bakken Formation of the Williston Basin,and Triassic Yanchang Formation of the Ordos Basin are four typical unconventional hydrocarbon systems selected as case studies herein.In each case,the formation of sweet-spot intervals for unconventional hydrocarbon resources was controlled by the coupled sedimentation of different global or regional geological events,collectively resulting in a favorable environment for the production,preservation,and accumulation of organic matter,as well as for the generation,migration,accumulation,and exploitation of hydrocarbons.Unconventional petroleum sedimentology,which focuses on coupled sedimentation during dramatic environmental changes driven by major geological events,is key to improve the understanding of the formation and distribution of sweet-spot intervals(areas)in unconventional petroleum systems.

Guadalupian(Middle Permian)δ^13Corg changes in the Lower Yangtze,South China

The Middle Permian Guadalupian witnessed significant environmental changes in the Phanerozoic such as large-scale sea-level drop,supercontinent Pangaea assembly,and transition from Early Permian glaciation to Late Permian non-glacial intervals.Carbonisotope tracers can provide insights for these environmental changes.Theδ^13C studies of the entire Guadalupian epoch are rare,and most of them has focused on near the end of Guadalupian and carb on isotopes of ino ragnics.Here,we present carbon isotopic compositions of organic matters in the Guadalupian from two sections(Chaohu and Xiaolao)in the Lower Yangtze area.South China.Our results show that 313Corg profiles in the Guadalupian show a peak in the Roadian and a gradual negative shift from the Roadian to the middle Capitanian.These trends can be matched by(δ^13C changes of carbonate rocks or organic matter in South China and other places in the world,representi ng a global carb on cycle signal.The Roadian positive peak was probably due to high productivity which was caused by upwelling during cooling time.The gradual negative shift of(δ^13C was caused mainly by a decrease of organic matter burial on land and in the ocean,resulting from global sea-level drop and anoxia-caused benthos decline,respectively.The less important causes for the gradualδ^13C negative shift are volcanic-gases releasing,decreased mountain belts,and the resultant reduced silicate weathering-consumption of CO2.The gradual negative shift ofδ^13C coincides with the gradual extinction in the Guadalupian.Therefore,global sea-level drop and marine reducing conditions may be the main causes of the gradual extinction in the Guadalupian.

The origin of bedding-parallel fibrous calcite veins in the Lower Permian Chihsia Formation in western Hubei Province,South China

Bedding-parallel fibrous veins occurring as lenticular to flattened intercalations were found in the organic-rich marlstone/calcareous shale of the upper Lower Permian Chihsia Formation in western Hubei Province,South China. They dominantly consist of fibrous calcite crystals with smooth and tight boundaries, forming fencelike inward, syntaxial growth clusters toward the vein center along which a median suture line generally occurs.Petrographic evidence indicates that these veins may form at relatively shallow burial depth, where fluid overpressures would have incrementally created the bed-parallel vein space, resulting in displacive growth of fibrous calcite.On the other hand, the C, O and S isotopic data across the vein reveal slightly depleted d13Ccarbvalues(-3.32 % to +0.19 % VPDB) and moderately depleted d18Ocarbvalues(-9.6 % to-7.3 % VPDB) with respect to those of coeval seawaters and slightly heavier d34Spyritevalues(-7.88 % CDT) with respect to those of ambient rocks.Stable isotope evidence consistently suggests significant contribution of bacterial sulfate reduction(BSR) to the formation of the fibrous calcite cements in the vein. The BSR could have been intensive with the availabilities of residual sulfate and abundant organic matters in theChihsia sediments during shallow burial, increasing the alkalinity of pore waters and further promoting carbonate precipitation. Thus, the bedding-parallel fibrous calcite vein in the upper Lower Permian Chihsia Formation is an important time-specific petrographic capsule, providing clues for understanding the diagenetic process in organicrich sediments.

A Kungurian Oceanic Upwelling on Yangtze Platform: Evidenced by δ^(13) C_(org) and Authigenic Silica in the Lower Chihsia Formation of Enshi Section in South China

The Late Paleozoic Ice Age across Carboniferous and Permian had a significant impact on the Kungurian （Upper Cisuralian series of Permian） Chihsia Formation in South China. This re- suited in a unique interval with features such as the lack of reef in Chihsian limestone, widespread stinkstone and nodular/bedded chert. The Chihsia limestone （Kungurian stage） deposited during a time of cooling was resulted from oceanic upwelling. Here we present evidence for this upwelling using sev- eral geochemical analyses： bulk organic carbon isotope, biomarker molecular geochemical data, and authigenic silica of the stinkstone member in the lower Chihsia Formation of the Kuangurian stage from the Enshi Section in western Hubei Province, South China. The lower part of the stinkstone member shows a rapid organic carbon isotope excursion with a -3%o shift triggered by the upwelling of 13C-depleted bottom water. The concurrent rapid increasing of authigenic silica content resulted from the enhanced supply of dissolved silica in the upwelling water mass. This upwelling at the Enshi Section also led to relative high TOC content, accounting for the widespread stinkstone in the lower Chihsia Formation during the Kungurian stage in Permian.

A Toarcian Ocean Anoxic Event record from an open-ocean setting in the eastern Tethys: Implications for global climatic change and regional environmental perturbation

The Early Toarcian“Oceanic Anoxic Event”(T-OAE)is recorded by marked disruption to both the climate system and marine ecosystems.Here,we present intergraded high-resolution carbon-isotope data(δ^(13)C),bulk geochemistry,mineral characterization from an open-ocean setting in the eastern Tethys.With these data,we(1)construct the high-resolution record of the T-OAE from an open-ocean setting in the eastern Tethys;(2)show that the T-OAE in the Sewa succession was marked by coarser-grained deposits associated with high-energy conditions within the otherwise low-energy claystone deposits that likely linked to a globally increased supply of clastic sediments into marginal and deeper marine basin;(3)propose that the low C_(org):P_(total) ratios,in combination with bioturbated structure and depletion or slight enrichment in redox-sensitive trace elements of V,Mo,and U suggest a long-term oxygenation event throughout the T-OAE interval at the Sewa succession,and hence,anoxia may not play a fundamental role during the Toarcian negative CIE in this setting;(4)exhibit that a warming and more humid climate began at the start of the T-OAE,and many episodic changes in sediment provenance throughout the T-OAE interval occurred at this location;and(5)suggest that accumulation of organic-matter sediments during the T-OAE is generally controlled by global climatic changes,but a regional environmental perturbation also might influence the preservation of organic matter.