Xiucheng Tan

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.

Permian–Triassic boundary microbialites (PTBMs) in southwest China: implications for paleoenvironment reconstruction

Permian–Triassic boundary microbialites (PTBMs) are commonly interpreted to be a sedimentary response to upwelling of anoxic alkaline seawater and indicate a harsh marine environment in the Permian–Triassic transition. However, recent studies propose that PTBMs may instead be developed in an oxic environment, therefore necessitating the need to reassess the paleoenvironment of formation of PTBMs. This paper is an integrated study of the PTBM sequence at Yudongzi, northwest Sichuan Basin, which is one of the thickest units of PTBMs in south China. Analysis of conodont biostratigraphy, mega- to microscopic microbialite structures, stratigraphic variations in abundance and size of metazoan fossils, and total organic carbon (TOC) and total sulfur (TS) contents within the PTBM reveals the following results: (1) the microbialites occur mainly in the Hindeodus parvus Zone but may cross the Permian–Triassic boundary, and are comprised of, from bottom to top: lamellar thrombolites, dendritic thrombolites and lamellar-reticular thrombolites; (2) most metazoan fossils of the microbialite succession increase in abundance upsection, so does the sizes of bivalve and brachiopod fossils; (3) TOC and TS values of microbialites account respectively for 0.07 and 0.31 wt% on average, both of which are very low. The combination of increase in abundance and size of metazoan fossils upsection, together with the low TOC and TS contents, is evidence that the Yudongzi PTBMs developed in oxic seawater. We thus dispute the previous view, at least for the Chinese sequences, of low-oxygen seawater for microbialite growth, and question whether it is now appropriate to associate PTBMs with anoxic, harsh environments associated with the end-Permian extinction. Instead, we interpret those conditions as fully oxygenated.

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.

Characteristics and genesis of the Feixianguan Formation oolitic shoal reservoir, Puguang gas field, Sichuan Basin, China

The Lower Triassic Feixianguan Formation at the well-known Puguang gasfield in the northeastern Sichuan Basin of southwest China produces a representative oolitic reservoir, which has been the biggest marine-sourced gasfield so far in China (discovered in 2003 with proven gas reserves greater than 350×108 m3). This study combines core, thin section, and scanning electron microscopy observations, and geochemical analysis (C, O, and Sr isotopes) in order to investigate the basic characteristics and formation mechanisms of the reservoir. Observations indicate that platform margin oolitic dolomites are the most important reservoir rocks. Porosity is dominated by intergranular and intragranular solution, and moldic pore. The dolomites are characterized by medium porosity and permeability, averaging at approximately 9% and 29.7 mD, respectively. 87Sr/86Sr (0.707536–0.707934) and δ13CPDB (1.8‰–3.5‰) isotopic values indicate that the dolomitization fluid is predominantly concentrated seawater by evaporation, and the main mechanism for the oolitic dolomite formation is seepage reflux at an early stage of eodiagenesis. Both sedimentation and diagenesis (e.g., dolomitization and dissolution) have led to the formation of high-quality rocks to different degrees. Dolomite formation may have little contribution, karst may have had both positive and negative influences, and burial dissolution-TSR (thermochemical sulfate reduction) may not impact widely. The preservation of primary intergranular pores and dissolution by meteoric or mixed waters at the early stage of eogenesis are the main influences. This study may assist oil and gas exploration activities in the Puguang area and in other areas with dolomitic reservoirs.

Mega-shoaling in carbonate platform of the Middle Triassic Leikoupo Formation, Sichuan Basin, southwest China

Shoaling is a common type of sedimentation in the evolution of carbonate platform, and commonly has poor continuity. This paper presents a newly discovered and rare type of shoaling, i.e., mega-shoaling in nearly basin scale, which is developed in the Middle Triassic Leikoupo Formation of the Sichuan Basin, southwest China. During the Leikoupo time, the studied Sichuan Basin experienced hot and dry climate conditions and developed a carbonate platform within a restricted epicontinental sea. In B sub-layer of the Lei-1-1 sub-member of the Leikoupo Formation a series of grainstones of shoal facies accumulated throughout almost the entire basin, thereby generating features associated with basin-scale mega-shoaling. By detailed core examination and microscopic observation of thin sections, it is shown that the lithology of this set of grainstones is dominated by doloarenite (calcarenite) followed by oolitic dolomite (limestone). In addition, it contains three types of sedimentary sequences characterized by upward-coarsening and upward-shallowing as the followings: restricted lagoon to platform interior beach; restricted lagoon to platform interior beach and to platform flat; and tidal flat to peritidal beach. Subsequently, a multicyclic stratigraphic division and correlation revealed that this set of grainstones can be well traced and compared horizontally, and is generally isochronous. In addition, a template for logging facies, established based on core calibrations and logging data, was employed to analyze the 235 wells in the basin. The results demonstrate the shoal grainstones to be 10–40 m thick with a 15×104 km2 continuous distribution area. These findings indicate that the carbonate platform developed mega-shoals within a short period of time. The genesis of such a mega-shoaling was investigated by focusing on various shoaling conditions, such as paleo-tectonics, paleo-geomorphology, paleo-climate, sea-level changes, and palaeo-hydrodynamics. A specific combination of independent geological factors creates beneficial geomorphologic conditions for the mega-shoaling including a quiescent paleo-tectonic environment, relatively flat paleo-geomorphology and evaporites filling up and leveling off. In addition, a stably settling carbonate platform underwent sea-level fluctuations through swift transgressions and protracted regressions, which is not only conducive to continuous, multicyclic and superimposed vertical development of grain beaches but also beneficial for the horizontal migration, coalescence and superimposition of individual grain beaches. As a consequence, large-scale and continuously-distributed grain beach sedimentation emerges and mega-shoals develop.

Comparison of basic features and origins of oolitic shoal reservoirs between carbonate platform interior and platform margin locations in the Lower Triassic Feixianguan Formation of the Sichuan Basin, southwest China

The oolitic shoal reservoirs of the Lower Triassic Feixianguan Formation carbonates in the Sichuan Basin of southwest China are an important target for gas exploration in the basin. Their occurrence, like other cases worldwide, can be divided into two locations in general, i.e., platform interior and platform margin locations. Their differences of reservoir features and origins, however, have not been investigated comprehensively due to different exploration degrees. This issue is addressed in this paper, to provide basic data and information for the basin’s hydrocarbon exploration and for the study of carbonate platform sedimentology and reservoir geology worldwide. We compared the features of these two types of reservoirs in detail, including the depositional and diagenetic features, pore types and petrophysical features. Based on the comparison, the origin of the reservoirs was further discussed. It is shown that the reservoirs in platform interior and platform margin locations differ significantly. The interior carbonates were deposited in moderate to high energy settings and the dominant lithologic type was limestone, which was weakly compacted and intensely cemented and has undergone meteoric dissolution. Pore types include intragranular dissolution and moldic pores, with low porosities (<6%) and low permeabilities (<0.1 mD). By contrast, the platform margin carbonates were deposited in relatively high energy settings and mainly consisted of dolostones with some limestones. The rocks were strongly compacted but incompletely cemented. As a result, some primary intergranular pores were preserved. Both meteoric solution and burial solution have taken place. There are various types of pore spaces including intergranular and intercrystalline solution pores and residual intergranular pores. This type of reservoir generally has better petrophysical properties (>9% porosity and >0.1 mD permeability) and pore-throat structures than the interior reservoirs. These differences were influenced by both primary depositional features and secondary diagenesis. For the interior carbonate reservoirs, early meteoric dissolution, weak compaction and strong cementation are important controlling factors. By contrast, the factors controlling the formation of the margin carbonate reservoirs mainly include dolomitization, preservation of primary pores and burial dissolution.

Restoration of paleokarst landform and its geological significance: A case from Middle Permian Maokou Formation in Northwestern Sichuan Basin

Abstract Based on the analysis of deposition and tectonism, “residual thickness method” is used to restore the paleokarst landform of Middle Permian Maokou Formation in northwestern Sichuan Basin. With the feature of plain with karst hilllock, the paleokarst landform in this area can be classified into three secondary geomorphic units: karst platform, karst slope and karst groove, in which the karst hillocks and monadnocks on karst platform and karst slope are the favorable zones for the development of karst reservoirs, and favorable exploration zones in the next step. Furthermore, in the karst grooves, the Maokou Formation are often denuded into Members Mao 3 or Mao 2, and the seismic profiles show the top of Maokou Formation in karst groove is missing due to erosion. Members Mao 4 and Mao 3 are generally preserved in the karst platform. The seismic profiles across the karst platform and karst groove show that the NE and NW striking erosion grooves were the result of differential uplift and erosion caused by basement faulting at the end of Middle Permian, which then successively developed and formed the NW striking Guangyuan-Wangcang and the NE striking Jiangyou-Guangyuan troughs in Changxing Period. It is suggested to pay more attention to the geological research and exploration of the shallow carbonate platform areas adjacent to the syneclise and trough in fairly deep water.

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.

Genesis and dolomitization of “Khali” powder crystal dolomite in Triassic Jialingjiang Formation, Moxi gas field, central Sichuan Basin, SW China

Abstract Taking the B bed of the second submember of the second member of Triassic Jialingjiang Formation in Moxi gas field as an example, the basic characteristics and controlling factors of the “Khali” powder dolomite reservoir is analyzed systematically, and the genesis and forming mechanism of the powder crystal dolomite are examined. Logging and core analysis show that the reservoir has intercrystalline pores and intercrystalline dissolved pores as the main storage space, mainly lamellar throats, good configuration between pores and throats, and medium porosity and low permeability, representing typical porous-type reservoir. Dolomitization genesis analysis indicates that dolomitization occurred in the early diagenetic stage, with obvious spatial evolution pattern, the degree of dolomitization decreases gradually from top to bottom, and decreases with the increase of distance from the source of brine horizontally; over-dolomitization occurs in the study area, the petrophysical property of dolomite close to the source of brine is worse than the dolomite formed by seepage-reflux far from the source of brine, which suggests the “Khali” powder crystal dolomite is of seepage-reflux genesis. Reservoir genesis analysis shows that the intercrystal pores formed by seepage-reflux dolomitization acted as superior seepage channels for reservoir reformation later, and syndiagenetic karstification is the root cause of the formation of “Khali” powder crystal dolomite reservoir.