The seismic property of reef flat in the Changxing–Feixianguan Formation, Northwest Sichuan Province, China

Abstract

The development of a carbonate reservoir is dominated by its primary sedimentary environment. Generally, a good reservoir must be the oolite of a bank or bioclastics of an organic reef in a marginal mesa, which must deposit on the slope of the mesa. Therefore, the analyses of sequence stratigraphy are the key for predicting the presence of carbonate reservoirs. A typical sequence pattern can be seen in the Puguang gas field. As an independent lithological body, the mechanical property of a reef flat is very different from the surrounding rock, causing stress to concentrate. Stress centralization will, in turn, cause a lot of fractures and faults to develop, and ultimately, result in a gas chimney, which is a seismic smear zone beneath or above the reef flat. Known gas fields, such as the Puguang gas field, also possess gas chimneys. Sequence stratigraphy patterns in the marginal mesa and gas chimney near the reservoir are very obvious in all known fields in southeast and east Sichuan Province. By analyzing the seismic features of all known gas fields, a model to predict carbonate reservoirs has been set up. Using this prediction model, we have found that there is a typical reef flat of the marginal mesa in the Malubei structure of Tongnanba structural belts whose sequence stratigraphy pattern is very much like that of the Puguang gas field; and in the Cangxi Jiulongshan structure, there is another reef flat which is symmetrical to the Malubei reef. The Cangxi reef flat possesses the largest platform edge shallow facies domestically found, and has a typical prograding sequence. Its gas chimney is very obvious too. Because the Cangxi reef flat is in the very vicinity of the hydrocarbon kitchen and the depression is much bigger than that of the Puguang gas fields, we predict that there may be a gigascopic gas field in Cangxi County. There may also be a large gas field in the Malubei structure.