Baomin Zhang

Formation, distribution, resource potential, and discovery of Sinian–Cambrian giant gas field, Sichuan Basin, SW China

Abstract The Anyue Sinian–Cambrian giant gas field was discovered in central paleo-uplift in the Sichuan Basin in 2013, which is a structural-lithological gas reservoir, with 779.9 km 2 proven gas-bearing area and 4 403.8×10 8 m 3 proven geological reserves in the Cambrian Longwangmiao Formation in Moxi Block, and the discovery implies it possesses trillion-cubic-meter reserves in the Sinian. Cambrian Formations in Sichuan Basin. The main understandings achieved are as follows: (1) Sinian–Cambrian sedimentary filling sequences and division evidence are redetermined; (2) During Late Sinian and Early Cambrian, “Deyang–Anyue” paleo-taphrogenic trough was successively developed and controlled the distribution of source rocks in the Lower-Cambrian, characterized by 20–160 m source rock thickness, TOC 1.7%–3.6% and R o 2.0%–3.5%; (3) Carbonate edge platform occurred in the Sinian Dengying Formation, and carbonate gentle slope platform occurred in the Longwangmiao Formation, with large-scale grain beach near the synsedimentary paleo- uplift; (4) Two types of gas-bearing reservoir, i.e. carbonate fracture-vug type in the Sinian Dengying Formation and dolomite pore type in the Cambrian Longwangmiao Formation, and superposition transformation of penecontemporaneous dolomitization and supergene karst formed high porosity-permeability reservoirs, with 3%–4% porosity and (1–6)×10 −3 μm 2 permeability in the Sinian Dengying Formation, and 4%–5% porosity and (1–5)×10 −3 μm 2 permeability in the Cambrian Longwangmiao Formation; (5) Large paleo-oil pool occurred in the core of the paleo-uplift during late Hercynian—Indosinian, with over 5 000 km 2 and (48–63)×10 8 t oil resources, and then in the Yanshanian period, in-situ crude oil cracked to generate gas and dispersive liquid hydrocarbons in deep slope cracked to generate gas, both of which provide sufficient gas for the giant gas field; (6) The formation and retention of the giant gas field is mainly controlled by paleo-taphrogenic trough, paleo-platform, paleo-oil pool cracking gas and paleo-uplift jointly; (7) Total gas resources of the Sinian–Cambrian giant gas field are preliminarily predicted to be about 5×10 12 m 3 , and the paleo-uplift and its slope, southern Sichuan Basin depression and deep formations of the high and steep structure belt in east Sichuan, are key exploration plays. The discovery of deep Anyue Sinian–Cambrian giant primay oil-cracking gas field in the Sichuan Basin, is the first in global ancient strata exploration, which is of great inspiration for extension of oil & gas discoveries for global middle-deep formations from Lower Paleozoic to Middle–Upper Proterozoic strata.

Geological conditions and distributional features of large-scale carbonate reservoirs onshore China

Abstract Based on well cores and thin section observations of more than 300 wells from major exploration target areas and formations in the Tarim, Sichuan and Ordos Basins, combined with seismic, well logging and testing data, the types and characteristics of carbonate reservoirs as well as the geologic conditions for their extensive development are analyzed systematically, and their distribution features are summarized. All varieties of marine carbonate reservoirs are developed in China, including three types of large-scale effective reservoirs, which are (1) depositional reef-shoal and dolomite reservoirs, (2) epigenetic dissolution-percolation reservoirs and (3) deep burial-hydrothermal altered reservoirs. Besides sedimentary facies, paleoclimate and paleogeomorphy, other factors controlling the development of deep large-scale effective reservoirs include interstratal and intrastratal dissolution-percolation and burial dolomitization which can be impacted by hydrothermal processes. Large effective reservoirs in deep carbonate rocks are distributed along unconformities and hiatuses in sedimentation, while reservoirs of epigenetic dissolution-percolation type extend from paleohigh uplift zones to lower slope reliefs. The reservoirs are widely distributed in stratified planar forms, and are superposed by multi-stage karstification processes vertically and have obvious heterogeneity controls. Burial dolomitization is restricted by primary sedimentary facies, and can form extensive effective reservoirs in deep layers in layered or stratified shapes. Hydrothermal related reservoirs are always distributed along deep, large faults, forming effective reservoirs in the form of a bead string in vertical direction and band-rod horizontally, which are not restricted by burial depth.

Formation, distribution and potential of deep hydrocarbon resources in China

Abstract The onshore exploration realm has been continuously expanded to (ultra-) deep oil and gas recently in China. New comprehension and significant breakthroughs have been made in understanding generation and preservation conditions, reservoirs formation mechanisms, exploration potential, petroleum resources assessments, and exploration engineering technology of deep oil and gas. Deep oil and gas reservoirs include clastic, carbonate and volcanic settings. The temperature of deep oil can be up to 295 °C. Long term shallow burial and rapid deep burial at late stages help preserve the porosity in deep clastic rocks, and dissolution and fracturing effects improve their reservoir properties. Affected by faulting, hydrothermal karst processes, dolomitization and early oil and gas injection, carbonate rocks have good reservoir properties even at depths of 8 000 m. Controlled by tectonism, volcanism, diagenesis and diagenetic reconstruction during supergene and burial stages, primary and secondary weathering types of reservoirs develop deep volcanic reservoirs. Deep oil and gas resources in China are distributed mainly within three main practical areas of carbonate, clastic and volcanic areas. Dominated by gas, some of the more productive areas include the Tarim, Ordos, Sichuan, Junggar, Songliao, Santanghu and Bohai Bay basins. Deep oil and gas exploration in China has entered an age of breakthrough and discovery. Relevant engineering technology, such as ultra deep well drilling and ultra high temperature drilling fluid techniques have facilitated the ability to find (ultra-) deep oil and gas.

Tectonic evolution from Late Sinian to Early Paleozoic and natural gas exploration in northwestern Sichuan Basin, SW China

Abstract Based on the field outcrops, drilling and seismic data of northwestern Sichuan Basin, this paper systematically discusses tectonic evolution characteristics from the Late Sinian to the Early Paleozoic, analyzes the petroleum geological significance, and points out the recent favorable exploration directions in this area. The area experienced mainly three times of tectonic evolution during the Late Sinian to the Early Paleozoic: (1) During the Late Sinian to the early period of Early Cambrian, this area was affected by tension action and formed two sets of nearly north-south trending continental rifts, and two sets of platform margins in the Sinian Dengying Formation. (2) During the middle period of the Early Cambrian to the Middle Ordovician, this area was eroded strongly (stronger in the northwest than in the southeast) because of multi-period uplifting of the northwestern Bikou paleo-land. (3) During the Late Ordovician to the Late Silurian, this area was eroded strongly again because of the compression of the Kwangsian Orogeny, and the erosion was stronger in the southwest than in the northeast. During the Late Sinian to the Early Cambrian, the formation of continental rifts controlled the development of bioherm beach facies reservoirs on the platform margins in the Dengying Formation and the high quality source rock in the Lower Cambrian series. During the middle of the Early Cambrian, the uplifting of the Bikou paleo-land controlled the distribution of the Kongmingdong Formation oolitic beach facies reservoirs around the paleo-land. It is suggested that, vertically, the fourth Member of the Dengying Formation and the Kongmingdong Formation be the main exploration targets, and, horizontally, the Jiulongshan structural trap be the most beneficial exploration belts recently due to its matching with petroleum migration and accumulation.

Origin, characteristics and significance of collapsed-paleocave systems in Sinian to Permian carbonate strata in Central Sichuan Basin, SW China

Abstract The origin, characteristics and geological significance of the collapsed-paleocave systems in the Sinian to Permian carbonate strata in Central Sichuan Basin are studied based on 3D seismic interpretation and attributes analysis, as well as imaging logging and cores. Collapsed-paleocave systems are distributed in the Sinian to Permian carbonate strata vertically and along the ends, releasing offsets and intersections of faults horizontally. On profile, they occur as seismic events push-down. The collapsed-paleocave systems mainly originated from fault movements at three main structural phases, the first stage of Tongwan, the second and third stages of Tongwan and the Early Caledonian, and eventually got into shape at the end of the Permian. The collapsed-paleocave systems are featured by multi-layers, wide distribution and good reservoir quality, in which reservoir space is made up of pre-existing pores, structure genetic fractures and collapsed genetic fractures. According to seismic response characteristics, three types of collapsed-paleocave systems are identified in this area, including multi-layer cave system collapsed complex, multi-layer cave passage collapsed system and intra-bed cave collapsed system. The multi-layer cave system collapsed complex and multi-layer cave passage collapsed system in the overlying deformed layers where fractures are widely distributed may be the favorable targets for future exploration.

Controlling effects of paleo-climate and paleo-ocean on formation of carbonate reservoirs

Abstract The controlling effects of paleo-climate and paleo-ocean on the formation of marine carbonate reservoirs were studied taking key carbonate strata in the Tarim, Sichuan and Ordos Basins as examples. Dry heat/aridity and moist heat/moisture are the two distinctive types of paleo-climate as far as carbonate sedimentation and reservoir formation are concerned. The geochemistry of the Phanerozoic ocean experienced cyclic variation, with calcite sea and aragonite sea occurring alternately. Carbonate exploration practice in the Tarim, Sichuan and Ordos Basins shows that the features of carbonate reservoir are evidently controlled by paleo-climate and paleo-ocean environment. The aragonite sea under dry heat/arid climate is most favorable for the formation of porous dolomite reservoirs. The calcite sea under dry heat/arid climate is favorable for the formation of porous dolomite reservoirs with evaporite moldic pores. The calcite sea and aragonite sea under moist heat/moist climate may only lead to the formation of cavernous carbonate reservoir. Paleo-climate and paleo-ocean environment exert great influence on carbonate sedimentation and its diagenesis, and ultimately determine the porosity features and types of carbonate reservoirs.

Paleofluid restoration and its application in studies of reservoir forming: A case study of the Ordovician in Tarim Basin, NW China

Abstract Karst reservoir paleofluid types of the Ordovician formation in the Tarim Basin are restored based on the analysis of element boron of filled mud, the test of fluid inclusions homogenization temperature and salinity, liquid anion and hydroxyl isotope of filled calcite in vug-fractures, and regional geologic background. The origin of the karst reservoirs are analyzed on this basis. The element boron contents of mud filled in vug-fractures are less than 80 μg/g generally; fluid inclusions have different homogenization temperatures in different regions, and the salinities are in a large range. The HCO 3 − contents are high, and the Cl − and SO 4 2− contents are dispersive in the liquid component of the fluid inclusions. The hydroxyl isotope contents are relatively dispersive, with relatively negative δD value and positive δ 18 O value. This evidence shows that the paleofluid of the Ordovician was from supergene atmospheric freshwater, buried fresh-brackish mixed water, seawater or concentrated seawater, and buried brine from underlying dolomite or evaporate rock of the Cambrian formation. The main constructive diagenesis for the formation of vug-cave type reservoirs is erosion and dissolution caused by atmospheric freshwater, and the reservoirs have been subjected to thermal fluid reformation from underlying evaporate rock of the Cambrian during burial stage.

Upper Cambrian-Ordovician reservoir characteristics in Well Gucheng-4 area, Tarim Basin

Abstract Based on the data of regional geologic background, core, and thin section, this paper studies the characteristics of sedimentary facies and reservoirs of the Upper Cambrian-Ordovician in Well Gucheng-4, Tarim Basin, and points out the potential and targets of petroleum exploration in the Gucheng break slope. Collapsed breccias are deposited in platform margins of the Upper Cambrian; Ordovician Penglaiba, Yingshan, and Yijianfang Formations sediments are controlled by eophyton, and lime mud mound facies of shallow glacis of platform frontal margin; Strumae limestone is deposited in the Ordovician Tumuxiuke Formation; and slope continental shelf abyssal basin facies controll the sediments in the Ordovician Queerqueke Formation. The Upper Cambrian dolomite is a better reservoir, which is controlled by deposition environment, dolomitization, and structural fractures. The Ordovician limestone reservoir is poor because of most primary pores being filled by cementation. The favorable exploration targets may lie in the north and west of the Well Gucheng-4 area.