Breakthrough of natural gas exploration in the beach facies porous dolomite reservoir of Middle Permian Maokou Formation in the Sichuan Basin and its implications: A case study of the tridimensional exploration of Well JT1 in the central-northern Sichuan Basin

Abstract

The neritic carbonate reservoir of Middle Permian Maokou Formation in the Sichuan Basin has been always treated as a fractured and fractured-vuggy reservoir. After realizing the breakthrough of natural gas exploration in the Lower Cambrian Canglangpu Formation in the Taihe gas bearing area of central-northern Sichuan Basin, Well JT1, a wildcat well, recently discovers a beach facies porous dolomite reservoir in the Maokou Formation above the Canglangpu Formation, and the high-yield industrial gas flow of 112.8 × 104 m3/d is obtained during the test. And thus, Well JT1 achieves the breakthrough of tridimensional natural gas exploration in multiple gas bearing series of strata in the vertical direction and realizes the new discovery of reservoir type in the Maokou Formation. In order to further clarify the characteristics and control factors of the beach facies porous dolomite reservoir of Maokou Formation in the Taihe gas bearing area, this paper studied its sedimentary evolution, high-energy beach facies characteristics, reservoir characteristics and following natural gas exploration direction. And the following research results were obtained. First, along the syneclise margin of central-northern Sichuan Basin develops a beach facies porous reservoir of the second Member of Maokou Formation, whose lithology is dominated by residual dolarenite and calcsparite calcarenite and reservoir space is mainly acted by intergranular pores and intercrystalline pores, with the average porosity of 4.9%. Second, in the middle and late stage of Maokou period, Guangyuan-Bazhong paleo faulted depression was developed in the northern Sichuan Basin and intraplatform low was developed in the central-northern Sichuan Basin. In the central and the northern Sichuan Basin, the high-energy beach of Maokou Formation is distributed around the faulted depression and the syneclise margin in a large scale, and the paleogeographic pattern of “one-margin and three-high belt” is presented. Third, under the control of high-energy beach facies, the Maokou Formation porous reservoir in the central-northern Sichuan Basin is distributed along the “one-margin and three-high belt” in a large scale. The high-energy beach facies of “one-margin and three-high belt” covers the area of 6500 km2. In this area, multiple sets of source rocks (Cambrian, Silurian and Permian) are developed vertically, hydrocarbon source fault is developed and carrier system is good, which is favorable for the large-scale hydrocarbon enrichment and accumulation. In conclusion, there are geological conditions for the development of facies controlled porous reservoir in the Maokou Formation of the Sichuan Basin, and the “one-margin and three-high belt” is the favorable area for the exploration of beach facies porous dolomite reservoir. The research results point out the new exploration field and direction for the large-scale increase of natural gas reserves in the Sichuan Basin.