Prediction of weathering paleokarst reservoirs by combining paleokarst landform with unconformity: A case study of Sinian Dengying Formation in Leshan-Longnüsi paleo-uplift, Sichuan Basin

Based on the research of unconformity type and distribution, and restoration of structural feature of paleo-uplift in the late sedimentary period of Sinian Dengying Formation, the evolution of paleokarst landform and karstic hydrodynamic field during depositional hiatus between the Cambrian and Sinian of Leshan-Longnüsi paleo-uplift is revealed to predict weathering paleokarst reservoir of Dengying Formation dynamically and effectively. The restoration of ancient structures shows that salient structures were developed continuously in the west of Weiyuan-Renshou, forming concurrent erosional-overlap unconformity, but in the east of Weiyuan- Renshou line, the palaeostructure high shifted from east of Zizhong-Yingshan to the west, forming inversion erosional-overlap unconformity. On this basis, the ancient karstic hydrodynamic field change caused by the change in paleogeomorphology was analyzed to find out the modes and strength of dissolution for different paleokarst landforms. The study indicates that areas along Hechuan-Dazu, Peng'an-Guang'an-Anyue-Neijiang and north of Renshou experienced strong dissolution, developing multiple sets of karst layers; Dazu-Tongnan-Hechuan area underwent fairly strong dissolution, resulting in fairly developed karst reservoirs; the two ends of the area west of Weiyuan-Lezhi-Xichong-Yilong also experienced certain degree of dissolution, with fairly developed karst reservoirs, whereas the middle of this area experienced weak dissolution, resulting in poor development of karst reservoirs. Key words: weathering paleokarst reservoir, paleokarst landform, unconformity, karst hydrodynamic field, Dengying Formation, Leshan-Longnüsi paleo-uplif

Tectonic evolution of Caledonian Palaeohigh in the Sichuan Basin and its relationship with hydrocarbon accumulation

The Caledonian Palaeohigh is an important gas exploration domain of Sinian and Lower Paleozoic in western Central Sichuan Basin where gas discoveries have been made successively in recent years. In order to sort out the relationship between the tectonic evolution of this Palaeohigh and hydrocarbon accumulation there, we carried out a new round of research based on previous study results. The evolution history of this Palaeohigh can be divided into seven episodes: the Late Sinian overall tilting and the youth form development of this Palaeohigh, the Cambrian-Ordovician syndepositional uplifting, the Silurian joint uplifting, the Devonian-Carboniferous uplifting and erosion, the Permian overall subsidence and deposition of regional caprock, the Triassic-Jurassic migration of structural high of the eastern segment of this Palaeohigh, and Cretaceous-Neogene strong deformation of the western segment of this Palaeohigh. The hydrocarbon accumulation in the Sinian-Lower Paleozoic experienced three evolution stages, namely the formation of ancient oil reservoirs in the Silurian-Triassic period, the development of the paleo-gas reservoirs in the Jurassic-Oligocene period, and differential evolution of gas reservoirs since the Miocene era. The Sinian-Lower Paleozoic paleo-reservoirs mainly occur in the Leshan-Ziyang-Gaoshiti-Longnǚsi zone and in the area to its north. The tectonic movement of the western segment of the Palaeohigh was strong, while its eastern segment was relatively stable since the later Himalayan epoch, which is favorable for gas accumulation and preservation. It is believed that the Gaoshiti-Moxi-Longnǚsi structural belt and its northern flank are not only favorable for the development of structural gas reservoirs in the Lower Paleozoic but also for gas reservoirs of karstic-lithologic type on the top of the Cambrian and Ordovician. The later type will be the major exploration target of this area in the future

Sedimentary Environment and Organic Accumulation of the Ordovician–Silurian Black Shale in Weiyuan, Sichuan Basin, China

The sedimentary environment and organic matter enrichment relationship of the Upper Ordovician Wufeng Formation black shale, Guanyinqiao mudstone, and Lower Silurian Longmaxi Formation black shale in the Sichuan Basin of Weiyuan are analyzed using geochemical methods such as organic carbon, sulfur, major elements, and trace elements. The experimental results illustrate that the upper section of the Wufeng Formation and the lower section of the Longmaxi Formation are organic matter enrichment layers. The presence of P indicates a high productivity level in the Sichuan Basin from the Late Ordovician to the Early Silurian. In addition, indicators such as V/Cr, Ni/Co, and S/C suggest that the Wufeng Formation was deposited under anoxic reductive conditions, that the ice age Guanyinqiao segment was in an oxygen-rich to oxygen-poor environment, and that the Longmaxi Formation was in a sulfidic environment. Mo/TOC indicates that the Wufeng Formation shale was controlled by a restricted basin and that the Guanyinqiao segment and the Longmaxi Formation were in a medium-to-weak retention environment. The weak correlation of TOC with P/Al and Al indicates that the level of primary productivity and terrigenous detritus had a minor effect on the organic matter enrichment of the Wufeng and Longmaxi Formation black shale. Conversely, the positive correlation of TOC with V/Cr and Ni/Co illustrates that the anoxic reductive sedimentary environment is the main factor affecting the organic matter enrichment of the Wufeng and Longmaxi Formation black shale. Based on these studies, the development model of organic-rich shales of the Ordovician–Silurian in Weiyuan, Sichuan Basin is proposed. This paper may provide a reference for shale gas exploration in the Wufeng–Longmaxi Formation and a sedimentary response to the major geological events of Ordovician–Silurian

Fractal Dimension Analysis of Pores in Coal Reservoir and Their Impact on Petrophysical Properties: A Case Study in the Province of Guizhou, SW China

Coal is a complex, porous medium with pore structures of various sizes. Therefore, it is difficult to accurately describe the characteristics of pore structure by using the traditional geometry method. The results from the present investigation suggest that the porous media system of the coal reservoir has obvious fractal characteristics at different scales. To study the complexity of the pores in the coal reservoir, 27 coal samples from Guizhou, SW China were studied. The fractal dimensions of coal pores were calculated, and the fractal dimension of a pore in a coal reservoir can be classified into two types: percolation and diffusion. The comprehensive fractal dimension can be obtained using the weighted summation method and the pore volume fraction of different fractal segments as the weight. The percolation fractal dimensions (Dp) of coal samples are between 2.88 and 3.12, the diffusion fractal dimensions (Dd) are between 3.57 and 3.84, and the comprehensive fractal dimensions (Dt) are between 3.05 and 3.63. The Dd values of all coal samples are all larger than the Dp values, which indicates that the random distribution and complexity of diffusion pores in coal are stronger than those of the percolation pores. The percolation fractal dimension decreases as the maturity degree increases, whereas the diffusion and comprehensive fractal dimensions increase. The diffusion pore volume fraction and total pore volume are all highly correlated with the comprehensive and diffusion fractal dimensions, respectively. The correlation between the comprehensive fractal dimension, diffusion pore volume fraction, and coal reservoir porosity is negative exponential, whereas the correlation between the total pore volume and coal reservoir porosity is positive linear. In comparison with the percolation and diffusion fractal dimensions, the comprehensive fractal dimension is better suited for characterizing the permeability of coal reservoirs. The fractal analysis of this paper is beneficial for understanding the relationship between the fractal characteristics of coal pores and properties