Characteristics and exploration prospect of dolograinstone beach reservoir in Xixiangchi Formation, Cambrian, Sichuan Basin

The Cambrian Xixiangchi Formation is an important succeeding stratum for gas explorationJP in the Lower Paleozoic of the Sichuan Basin, for which, the exploration degree is low, and the main controlling factors of large-scaleJP reservoirs have not been well understood. Based on core, logging, seismic data and combined with regional tectonic evolution analysis, the original sedimentary structure of the shoal dolograinstone reservoir in the CambrianJP Xixiangchi Formation were restored in this paper, and the characteristics, controlling factors and development patterns were further discussed. Results show that: (1) The rock types of shoal dolograinstone karst reservoirs mainly include dolograinstone, dolopackstone, granulated dolostone, residual granular dolomite and grain dolomite. It appeared to be developed in a variety of sedimentary micro-facies assemblages, such as shoal-shoal superimposition, flat-shoal-flat, lagoon-beach-lagoon and flat-beach-lagoon. The main reservoir space are intergranular (dissolution) holes, intergranular (dissolution) pores and vugs, and a small numberJP of (dissolution)fractures were also developed. The reservoir porosity ranges from 3% to 6% with a highest value greater than 10%. (2) The dolograinstone shoal facies dolostone of the Xixiangchi Formation was controlled by the sea level changes and synsedimentary paleouplift. A large number of dissolution pores were formed after multiple exposure in the contemporaneous and quasi contemporaneous periods. Among them, the Yunan Movement at the end of the Xixiangchi period has a wide range of distribution and extensive exposure in the whole basin, which may further transform the middle and upper reservoirs of the Xixiangchi Formation. The Duyun and Guangxi movements might transform the central Sichuan paleouplift and its surrounding reservoirs. (3) The favorable reservoir distribution areas of Xixiangchi Formation in the Sichuan Basin were predicted to develop mainly type Ⅱ and Ⅲ reservoirs. Among them, the scale reservoir distribution area of the slope belt around the paleo-uplift is more than 10 000 km²JP, which is expected to be the most favorable exploration area at present.