Fine description of ramp-type small bioherms and breakthrough of “two bioherms in one well” in high-yield gas wells: A case study of the Changxing Formation small bioherm group in the eastern Sichuan Basin

Abstract

Ramp-type bioherm is geologically characterized by lateral migration, small size and discrete distribution. The fine bioherm description is of high difficulty and the benefit development of bioherm gas reservoirs can be hardly realized. To solve these difficulties, this paper put forward a new bioherm identification method based on well-seismic combination after cognizing the geological characteristics of ramp-type bioherms deeply. And this new method plays a successful role in guiding the drilling of Well Y012-X16 in the mode of “two bioherms in one well” in the Damaoping Block of eastern Sichuan Basin. High-yield industrial gas flow of 113.65 × 104 m3/d is obtained during the test and great new progress is achieved in the exploration of bioherm gas reservoirs. And the following research results were obtained. First, the degree of slope at the marginal platform slope of ramp-type bioherm is only 2°–4°. The migration and evolution laws of bioherms are mainly controlled by the change of paleogeomorphology and sea level. As a result, small bioherms of multiple rows and multiple periods are formed and the gas-water contact of ramp-type bioherm gas reservoir is not united, presenting the characteristics of “one bioherm, one reservoir”. The gas bearing property is good at the top of the bioherm. The overall gas bearing property is poorer in the early bioherm of the second member of Upper Permian Changxing Formation and better in the late bioherm of the third member of Changxing Formation. Second, a new bioherm identification method with “double-high” integrated seismic processing technology, data driven “box-shaped volume” perspective technology, remaining thickness method based sedimentary paleogeomorphology restoration technology and three-dimensional visualization technology as the core is developed innovatively, which provides the fine description of ramp-type small bioherms in the three dimensional space. Third, based on the application of the new bioherm identification method, the distribution detail of the bioherms in the Damaoping Block is clarified, the development mode of “two bioherms in one well” for long horizontal sections is established and a PetroChina's new record of bioherm reservoir drilling length is set up in Well Y012-X16, which launches a new means to the benefit development of small bioherm group. Fourth, it is predicted that a number of bioherm development areas can be newly increased in the Damaoping Block, which can provide the supplement productivity of 100 × 104 m3/d for the development of gas reservoirs, so their natural gas exploration and development potential is huge. In conclusion, the fine description technologies for ramp-type small bioherms are conducive to the identification of ramp-type small bioherm group and the efficient exploration and development of bioherm gas reservoirs in the Sichuan Basin.