Bing Luo

Development and reservoir significance of mud mounds in Sinian Dengying Formation, Sichuan Basin

Abstract Based on core and outcrop profile observation, the characteristics and reservoir significance in oil and gas accumulation of mud mounds in the Sinian Dengying Formation of the Sichuan Basin were studied. Small-scale mud mounds in platform are widespread in the Sinian Dengying Formation, Sichuan Basin, mostly less than 2 m thick per cycle, but high in occurrence frequency, especially in Member 2 and Member 4 of the Sinian Dengying Formation. Particles constituting the mud mounds are thrombolite, algal stromatolite, oncolite and bindstone, related to cyanobacteria biocoenosis buildup, which bind and capture lime mud for the mud mound construction, and the relevant rock types include algal bound clotted dolomitite, algal bound frame-dolomitite, and undulating-cylindrical algal stromatolite associated with clotted dolomitite. Multiphase mud mounds with positive relief have abundant frame structures, zebra structures and stromatactis. Two kinds of depositional models are mound base-core-flat and mound base-core-cap, the former is more common. The mud mounds are closely related to reservoirs. Different sizes of pores developed in the mud mound core provide space for oil and gas accumulation, so the mound-shoal complex is the main exploration target of the Sinian Dengying Formation.

Hydrocarbon accumulation of Sinian natural gas reservoirs, Leshan-Longnüsi paleohigh, Sichuan Basin, SW China

Abstract The old Sinian carbonate reservoir in the Leshan-Longnusi paleohigh was taken as a research object to study the process of gas accumulation in the Sinian reservoir through analysis of gas reservoir characteristics, gas composition, gas reservoir types, accumulation condition and evolution. The results show that the reservoir lithology and type are almost the same in the six gas pools discovered in the Leshan-Longnusi paleohigh. All the gas reservoirs are characterized by high temperature, ordinary pressure, and intense heterogeneity. The gas reservoir type in different layers and the gas compositions and carbon isotopes in different locations vary obviously. The gas of Sinian Dengying Formation, originated from oil cracking, is mixed gas mainly from source rocks of Sinian Dengying Formation as well as Cambrian Qiongzhusi Formation. The source and reservoir condition, their combination and fluid transporting conditions are favorable, which can determine the gas accumulation and preservation in Dengying Formation. The Sinian gas reservoirs are believed to have been accumulated by the following processes: paleo-oil accumulation, paleo-oil cracking, and gas reservoir adjustment and finalization. There are three processes of gas accumulation in the reservoir, which are influenced by the formation of paleohigh and differential structural evolution in different positions.

Basin-scale sand deposition in the Upper Triassic Xujiahe formation of the Sichuan Basin, Southwest China: Sedimentary framework and conceptual model

The Upper Triassic Xujiahe (须家河) Formation in the Sichuan (四川) Basin, Southwest China is distinctive for the basin-scale sand deposition. This relatively rare sedimentary phenomenon has not been well interpreted. Here we addressed this issue by discussing sedimentary framework and conceptual model. Analysis of sedimentary setting implied that the basin received transgression during the deposition. It had multiple provenance supplies and river networks, as being surrounded by oldlands in multiple directions including the north, east and south. Thus, the basin was generally characterized by coastal and widely open and shallow lacustrine deposition during the Late Triassic Xujiahe period. This is similar to the modern well-known Poyang (鄱阳) Lake. Therefore, we investigated the framework and conceptual model of the Sichuan Basin during the Xujiahe period with an analogue to the Poyang Lake. Results show that the conceptual model of the deposition can be divided into transgressive and regressive stages. The first, third and fifth members of the formation are in transgressive stage and the deposits are dominated by shore and shallow lacustrine mud. In contrast, the deposition is mainly of braided river channel sand deposits during the regressive stage, mainly including the second, fourth and sixth members of the formation. The sand deposited in almost the entire basin because of the lateral migration and forward moving of the cross networks of the braided rivers. The multiple alternations of short and rapid transgression and relatively long regression are beneficial to the basin-scale sand deposition. Thus, the main channel of the braided river and its extensional areas are favorable for the development of hydrocarbon reservoir. This provides practical significance to the reservoir evaluation and exploration. In addition, the results also justify the relatively distinctive sedimentary phenomenon in the study area and may also have implications for understanding the large-scale sand deposition elsewhere.

Structural characteristics of intra-platform shoal in the Leikoupo Formation in the transitional zone of the central and southern Sichuan Basin

Abstract Through comprehensive research on drilling, logging and core data of T 2 l 1 1 sub-member, which is the major pay of the Leikoupo Formation in the transitional zone of the central and southern Sichuan Basin, the structural characteristics, distribution regularity and its major controlling factors of intra-platform shoal are studied. The major rocks of intra-platform shoal in T 2 l 1 1 sub-member include oosparite, residual oolitic dolostone, sparrenite, residual doloarenite and sparry doloarenite, sparry bioclastic limestone and residual bioclastic dolostone, which constitute oolitic shoal, arene shoal and bioclastic shoal. The intra-platform shoal is characterized vertically by shallowing-upward sequences, and consists of three sedimentary units, namely shoal base, shoal core and shoal cap. Besides, three microfacies can be distinguished laterally, shoal core, shoal edge and intershoal sea. The variation of water depth and hydrodynamic conditions caused by sea level change control the characteristics and superposed pattern of shoal body. Meanwhile, the palaeogeomorphology with alternating uplifts and depressions defines the differential distribution and lateral combination pattern of intra-platform shoal body.

Karst paleogeomorphology of the fourth Member of Sinian Dengying Formation in Gaoshiti-Moxi area, Sichuan Basin, SW China: Restoration and geological significance

Abstract Based on 3-D seismic data and drill-hole data in Moxi-Gaoshiti area, the impression seismic thickness from the top of Longwangmiao Formation, which is easier traced, to the bottom of Cambrian System was selected to characterize the karst paleogeomorphology of the fourth Member of Dengying Formation. The paleogeomorphology of the member can be further divided into three geomorphic units: karst platform, slope and superimposed slope, which had different paleohydrologic conditions, and thus different karstification intensity and reservoir quality. Among them, the superimposed slope with the strongest dissolution, has larger solution groove and cave systems, where the reservoirs with mainly honeycombed dissolved pores and cavities as storage space, are best in quality; the platform slope with weaker dissolution, has smaller solution groove and cave systems, where the reservoirs are poorer than those in the superimposed slope; the karst platform with the weakest dissolution, has piebald karst systems mainly, where the reservoirs with mainly pinhole to smaller dissolved pores and cavities as storage space, are poorer in quality on the whole, but there are some good quality reservoir bodies in local areas. The results show that, besides the highly explored scarp belt, the slope in the intra-platform with low exploration degree can also be favorable exploration area of the fourth member of Dengying Formation.

Differences between the platform-margin shoal reservoirs and the platform-interior shoal reservoirs of the Middle Triassic Leikoupo Formation, Sichuan Basin, China

The carbonate shoal grainstone reservoirs of the Middle Triassic Leikoupo Formation in the Sichuan Basin provide a typical example of gas exploration in shoal carbonate reservoirs. The carbonate shoal reservoirs in Leikoupo Formation are of two types: platform-margin shoals and platform-interior shoals. Based on a comprehensive data analysis, and through analysis and correlation of grain shoal reservoir characteristics, it has been shown that there are significant differences between platform-margin and platform-interior shoal reservoirs in terms of the types of reservoir space, the relationship between porosity and permeability, and the pore throat structures. The pores in the platform-margin shoal reservoirs are mainly relict intergranular pores and intergranular dissolution pores, and their porosity and permeability show a good positive correlation. The throats are thick and the pore throats have a good distribution. Isolated intragranular solution pores and moldic pores are mainly developed in the platform-interior shoal reservoirs. There is a weak positive correlation between porosity and permeability, pore throats are less developed, and the pore throats have poor configurations. Differences in sedimentary environment and diagenetic evolution are the primary causes of the different reservoir characteristics of the two shoal facies. The water body in the platform-margin shoal deposits had a higher energy than that in the platform-interior shoal deposits, and its wave base was deeper that of the platform-interior shoals. The shoal bodies at the platform margin are much thicker than those in the platform interior. The grainstones of platform-margin shoals are much better sorted. The rocks have undergone compaction during most post-depositional stages. The thicker grainstones of the platform-marginal shoals preserved their original intergranular pores better than the lower-thickness grainstones in the platform-interior shoals. This was favorable for formation of intergranular dissolution pores during later burial. In contrast, the grainstones of the platform-interior shoals are less thick and early intergranular cements are well developed, resulting in large-scale loss of the original intergranular pores. However, as a result of selective dissolution by early fresh water in the meteoric diagenetic environment, intragranular solution pores and moldic pores were readily formed. Thus, the platform-margin shoal reservoirs show preservation of the original intergranular pores enhanced by burial dissolution, but the platform-interior shoal reservoirs are mainly the result of early selective dissolution by meteoric fresh water.

Basic characteristics and accumulation mechanism of Sinian−Cambrian giant highly mature and oil-cracking gas reservoirs in the Sichuan Basin, SW China:

Old Mesoproterozoic−Cambrian successions have been regarded as an important frontier field for global oil and gas exploration in the 21st century. This has been confirmed by a recent natural gas exploration breakthrough in the Sinian and Cambrian strata, central Sichuan Uplift, Sichuan Basin of SW China. However, the accumulation mechanism and enrichment rule of these gases have not been well characterized. This was addressed in this work, with aims to provide important guidance for the further exploration while enriching the general studies of the oil and gas geology in the old Mesoproterozoic–Cambrian strata. Results show that the gas field in the study area is featured by old target layers (Sinian–Lower Cambrian), large burial depth (>4500 m), multiple gas-bearing intervals (the second and fourth members of the Sinian Dengying Formation and the Lower Cambrian Longwangmiao Formation), various gas reservoir types (structural type and structural–lithologic type), large scale (giant), and superimposing and ubiquitous distribution. The giant reserves could be attributed to the extensive intercalation of pervasive high quality source rocks and large-scale karst reservoirs, which enables a three-dimensional hydrocarbon migration and accumulation pattern. The origin of natural gas is oil cracking, and the three critical stages of accumulation include the formation of oil reservoirs in Triassic, the cracking of oil in Cretaceous, and the adjustment and reaccumulations in the Paleogene. The main controlling factor of oil and gas enrichment is the inherited development of large-scale stable paleo-uplift, and the high points in the eastern paleo-uplift are the favorable area for natural gas exploration.