Search

The Lower Cambrian Xiaoerbulake Formation consist of thick dolostones which are high-quality hydrocarbon reservoir in the western Tarim Basin, but the origin of dolomite is still controversial, which lead to poor understanding to the origin of reservoir beds. By using the latest core data of Well XKD-1, we analyzed the petrology and geochemistry of the formation in order to clarify the genesis of dolomite. The geochemical analysis includes Sr, C, and O isotopes, and in-situ minor element and rare Earth element measurements for different types of dolomites by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Four types of lithofacies are recognized in the Xiaoerbulake Formation based on different textures, which are laminated dolostone, thrombolite dolostone, bacteria-bonded dolostone, and grain dolostone. Two types of dolomites can be separated which are matrix dolomite (MD) and cement dolomite (CD). MD is the primary type of laminated and bacteria-bonded dolostone, and CD was developed mainly in grain dolostone and thrombolite dolostone. The δ13C and δ18O indicate that the grain dolostone and thrombolite dolostone have undergone more intensive diagenetic modification. On the contrary, the diagenetic modification of laminated and bacteria-bonded dolostone are relatively weak, which implies that the dolomitization of the latter two lithofacies occurs before burial stage. The 87Sr/86Sr ratio indicates that the corresponding fluid for dolomitization was seawater or marine-origin fluids. The MD was formed in the near-surface stage via the penecontemporaneous dolomitization of seawater or hyper-saline seawater. The dolomitization of CD occurred in the burial environment with the porosity was filled by marine-origin fluid, indicating a seepage-reflux dolomitization process. Both types of dolomites are partially affected by hydrothermal activities.

The paleotectonic pattern, lithofacies paleogeographic features, sedimentary evolution and its controlling effects on hydrocarbon accumulation assemblages during the depositional period of the Sinian Dengying Formation in middle-upper Yangtze region were investigated based on outcrops, drilling, log and seismic data. The study shows that, (1) Affected by the breakup of the Rodinia supercontinent, the middle and upper Yangtze areas were in extensional tectonic environment during the depositional period of Dengying Formation. The carbonate platform was structurally differentiated. Intra-platform depressions controlled by syndepositional faults developed, forming a tectonic-paleogeographic pattern of “three platforms with two depressions”. (2) During the depositional period of the first and second members of the Dengying Formation, rimmed platforms and intra-platform fault depressions developed in upper Yangtze area and isolated platform developed in middle Yangtze area, and there was the Xuanhan-Kaijiang ancient land block in eastern Sichuan. The depositional period of the third member of the Dengying Formation is the transformation period of tectonic-paleogeographic pattern, when a set of shallow water shelf sediment rich in mud was deposited due to transgression on the background of the eroded terrain formed in Episode I of Tongwan Movement. The sediment of the fourth member of the Dengying Formation inherited the paleogeographic pattern of the first and second members of the Dengying Formation in general, but the Deyang-Anyue intra-platform fault depression further expanded, and the middle Yangtze platform evolved into two separated platforms. (3) Tectonic-sedimentary differentiation and evolution of carbonate platform in the Sinian gave rise to two types of accumulation assemblages with wide distribution and great exploration potential, which are platform margin and intra-platform.

The concept of an intermingled fractal unit (IFU) model was first proposed by Atzeni and Pia in 2008, and their model has since been successfully applied to predict thermal conductivity, electrical conductivity, and the mechanical properties of porous media materials. This paper, based on the Pia IFU model, fits the pore size distribution spectrum to quantitatively characterize the Triassic Karamay Formation conglomerate reservoirs in the Mahu region, in the Junggar Basin of Northwest China, and makes permeability predictions using the free fluid T2 spectrum according to the nuclear magnetic resonance (NMR) experimental data. The results show that the accuracy of the IFU model is significantly higher than that of the classic Coates and SDR models for conglomerate reservoirs with complex pore structures, indicating that this is an effective method to calculate permeability based on NMR. In addition, preliminary discussions are entered into regarding the intermingled fractal expression of the Kozeny-Carman equation and the relative permeability, in order to widen the application of the IFU model in reservoir physics. The derived expressions appear complicated in form but are straightforward to calculate and apply using computer programming since their iteration parameters are definite. The findings set out in this paper provide a valuable reference for further research of the IFU model in reservoir physics.

Erosion of carbonate strata not only can reflect the karst landform, but also can reveal the characteristics of carbonate karst reservoirs indirectly. In Sichuan Basin, the quantity and root causes of lacuna in the top strata of the Maokou Fm are unclear. In this paper, the features of such stratigraphic lacuna were analyzed qualitatively. Then, in accordance with the spectral transformation of GR logs and the characteristics of cycle in the Maokou Fm penetrated by four typical wells, the Milankovich cycles of the logs were identified. Then they were used for quantitative calculation of erosion quantity and for highlighting the root causes for the stratigraphic lacuna. According to the research results, the Mao-4 Member is residual in the Yibin–Ya'an–Jiangyou area and the Shizhu area, but missing in the other parts of Sichuan Basin with a stratigraphic lacuna intensity gradually increasing from the southern part to the central part and to the northern parts of the basin. Besides, the stratigraphic lacuna of the Maokou Fm varies from 0 to 200 m – for example, 0–60 m in the southwestern and northeastern parts of the basin, and 140–200 m in the southern, central and northern parts of the basin. Moreover, formation denudation occurred at the end of the Maokou Period was induced predominantly by erosion generated as the sea level fell during the glacial age, especially in the northern part of the basin. Also, the Mid-Permian karst landform inherited the sedimentary features of being high in the southwest and low in the northeast. In the areas from the southwestern to central and to the northern Sichuan Basin, the karst landform transits from erosion highlands to karst upper and lower slopes, coinciding well with the sedimentary features of the Wujiaping Period. In conclusion, the Milankovich cycles can be used to calculate the stratigraphic erosion in carbonate formations efficiently and accurately. The technique can be extended to the restoration of stratigraphic erosion in other marine basins. Keywords: Sichuan basin, Mid-Permian, Maokou Fm, Milankovich cycle, Stratigraphic erosion, Quantitative calculation, Glacial age, Genesis discussion, Karst landform

The structure characteristics and major controlling factors of platform margin microbial reef reservoirs in the Lower Cambrian Xiaoerbulak Formation in the Aksu area, Tarim Basin were analyzed based on 5 outcrop sections, 162 thin sections, 12 SEM samples, 52 sets of porosity and permeability data. Macroscopically, small-scale microbial reefs form the platform margin. A single microbial reef has several microbial reef progradation complexes, including reef front, fore reef, reef crest, and back reef, but microscopically, they have different kinds of microstructures. The reservoir spaces in the reefs can be divided into microbial structure reservoir space, including fenestral, frame, moldic, and oversized dissolution pores, and non-microbial structure reservoir space, including microcracks and stylolites. The statistical results of porosity and permeability show that in the Yutixi Section, porosity and permeability of the microbial reef reservoirs are generally below 5% and 1.0×10−3 μm2 respectively, characteristics of extremely low porosity and permeability reservoirs; while the reef reservoirs in the Sugaitebulake Section have a wide porosity range between 3% and 10%, and permeability range between 0.1×10−3 μm2 and 50×10−3 μm2 and strong heterogeneity, are low-medium porosity, low-medium permeability reservoirs. This demonstrates different microbial reefs have big differences in physical properties, and even the reservoir in the same reef has obvious heterogeneity. Paleotopography controls the formation of microbial reefs and sedimentation controls the facies distribution and the primary porosity development. Dissolution, controlled by the two former factors, finally decides evolution of reservoir pores. Key words: microbial reef, thrombolites, laminites, reservoirs, rimmed platform, Tarim Basin, Lower Cambrian, Xiaoerblak Formation

A large inherited paleouplift from Pre-Sinian to Early Cambrian, named Xuanhan-Kaijiang paleouplift, has been discovered based on multiple geological information in the eastern Sichuan Basin: firstly, onlap deposition of Upper Sinian Dengying Formation and Lower Cambrian is observed from peripheral zone to central Xuanhan-Kaijiang area from the seismic profile; secondly, Dengying Formation thickness distribution map shows that this formation is thinnest in Xuanhan-Kaijiang area, and thickens towards the surrounding areas; thirdly, terrigenous clastic rock is observed in Upper Sinian Dengying Formation on outcrops in Chengkou County and Wuxi County etc., indicating the development of a paleouplift. The analysis of the characteristics, formation and evolution of the paleouplift reveals that the plane distribution of the paleouplift shows an arched structure, stretching in a north-south trend. The area of the paleouplift is around 16 000 km2. Vertically, the paleouplift can be divided into three parts: platform in the middle, steep slope in the west and ramp in the east. Originally, the paleouplift was an erosion-type uplift above the water level during the early stage of the Late Sinian; and gradually transformed into deposition-type uplift under water level during the late stage of the Late Sinian to the Early Cambrian. The formation of the paleouplift was controlled by several factors, including basement uplift, intracratonic rift, basement faults and regional uplifting. The paleouplift and its slope areas, favorable for the development of high energy facies and karst weathering reservoirs, are important exploration target areas in the eastern Sichuan Basin in the near future. Key words: Sichuan Basin, eastern Sichuan, paleouplift, Xuanhan-Kaijiang paleouplift, Sinian to Early Cambrian, intracratonic rift, onlap deposition

The Cambrian carbonate & evaporite paragenesis system in the Sichuan Basin is made up of the Longwangmiao, Gaotai and Xixiangchi Fms. So far, great breakthrough has been made only in the Longwangmiao Fm instead of the latter two, and the Anyue Gasfield was discovered in the center of this basin. In this paper, therefore, the Cambrian carbonate & evaporite paragenesis system in the Sichuan Basin was analyzed in terms of its structural–sedimentary setting, sequence stratigraphic framework, sedimentary facies and the distribution of evaporites by using various geologic, logging and seismic data. Then, the geological model of sedimentary facies was established and the distribution range of favorable reservoirs was predicted. Based on these studies, the following results are obtained. Firstly, the palaeotectonic framework is characterized by the style of “one depression between two uplifts” in the setting of a large SE dipping slope, and the stratigraphic filling is in the structure of “onlapping at the bottom and truncation at the top” which is thin in the west and thick in the east. Secondly, three third-order sequence cycles which, on the whole, become shallow upward are developed from bottom to top, and gypsum-salt rocks are mainly located at the high system tract (HST) of third-order sequences and concentrated in the Wanzhou–Yibin sag. Thirdly, the geological model of sedimentary facies is composed of three major sedimentary structural layers from bottom to top, namely the evaporative carbonate ramp, the evaporative diamictic restricted platform and the evaporative restricted platform. The sedimentary environment changes from the open to the closed and the penesaline for a long time, and then back to the open. The distribution of shoals changes from the pattern of “dual banks” in a large area to more scattered shoals and banded shoals, while the evaporative lagoon and tidal flat shrink. Fourthly, the reservoir distribution is complicated and mainly controlled by the sedimentary pattern of palaeotectonics, the grain shoals and the exposed surfaces of sequences with different orders. The favorable areas for reservoir development are located above the paleo-uplift and its surroundings, at the slope-break belt in the platform and in the periphery of platform margin.

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. Key words: Sichuan Basin, Late Sinian – Early Paleozoic, tectonic evolution, natural gas exploration, intracontinental rift, Bikou paleo-land, Dengying Formation, Kongmingdong Formation, Jiulongshan structure

The microbial carbonate reservoirs of the upper part of the Lower Cambrian Xiaoerbulak Formation have been researched based on outcrop section and thin section identification in the Aksu area, Tarim Basin. The microbialites are composed of peloid, lamination, thrombolite, stromatolite, dendrolite, oncolite and spongiostromata. There develop three types of microbial carbonate reservoirs: microbial reef, coated thrombolite and spongiostromata stromatolite dolomite. Two stages of microbial reefs are developed on paleokarst high in the Late Sinian: the first one with southward progradation is constructed by the Epiphyton, while the second one with northward transgression by cyanobacteria clots. The dendrolites, thrombolites and stromatolites in the reefs are potential for hydrocarbon reservoirs, with needle-like dissolved pores, thickness of 40 to 70 meters, lower porosity and lower permeability. Coated thrombolites are located on the paleo-depositional slope, characterized by the isolated dissolved pores, thickness of 30 meters, lower porosity and lower permeability. Spongiostromata stromatolite beds lie in the paleo-depositional depression, with layered fenestrules well developed, 18 meters thick, lower to medium porosity and lower permeability. The development of the microbial carbonate reservoirs are controlled by depositional paleo-topography, diagenesis and microbial structures. Key words: Lower Cambrian, microbialite, carbonate reservoir, microbial reef, thrombolite, stromatolite, Tarim Basin