Development models of Xixiangchi Formation karst reservoirs in the Late Caledonian in the central Sichuan Basin and its oil-gas exploration implications

Oil and gas shows are abundant in the process of drilling the Cambrian Xixiangchi Formation in the Sichuan Basin, so this formation is the reserve exploration area and the important replacement formation in this basin. At present, however, it is less explored and the distribution laws of its high-quality reservoirs are not understood sufficiently. In order to determine the next natural gas exploration directions and targets of Xixiangchi Formation in the central Sichuan Basin, this paper studied its reservoir characteristics and the distribution of karst unconformity using the well-seismic combination method, based on observation data of typical field outcrops, cores and thin sections. Then, a development model of the Xixiangchi Formation karst reservoirs in the Late Caledonian in this area was established, and the favorable development area of shoal-facies karst reservoir was predicted. And the following results were obtained. First, the Xixiangchi Formation reservoir is lithologically composed of grain dolomite and crystalline dolomite. The reservoir space is dominated by dissolved pores, intergranular pores, intercrystalline pores and fractures. It is a low porosity and low permeability fractured-porous reservoir with an average porosity of 3.46% and an average permeability of 0.99 mD. Second, in the Xixiangchi Formation, there is a ring-shaped denudation zone that is wide in the central part and narrow in the southern and northern sides, and 6-50 km in width and 4 700 km2 in area. Third, due to formation uplifting and denudation in the development area of denudation zone in the Late Caledonian, strong weathering crust karstification occurred in the Xixiangchi Formation, and two types of reservoirs were developed, including the shoal-facies superimposed supergene karst reservoir in the exposed denudation area and the shoal-facies superimposed bedding karst reservoir in the buried area. Fourth, the Xichong-Guang'an-Tongnan area is the most favorable area for the development of shoal-facies karst reservoir, covering an area of 5 000 km2. In conclusion, Xichong-Guang'an area is adjacent to the hydrocarbon generating center of Lower Cambrian Qiongzhusi Formation. In this area, hydrocarbon source faulting is developed, and the Xixiangchi Formation has large shoal bodies and strong karstification, so a large-scale effective reservoir of shoal facies superimposed Late Caledonian karstification tends to form. Therefore, it can be taken as a favorable target of natural gas exploration in the Xixiangchi Formation in the next step. © 2020, Natural Gas Industry Journal Agency. All right reserved.