Gypsum is important for hydrocarbon exploration in carbonate-evaporite basins, because it usually coexist with source and reservoir rocks. The plasticity of gypsum can cause the formation of tectonic traps, in addition with its good capping ability, effective assemblage of source-reservoir-caprock-trap may be formed. Leikoupo Formation in Longgang Area of Sichuan Basin belongs to evaporative platform to lagoon sediment system, which has large area of gypsum. Exploration achievements indicate that the discovered gas in Leikoupo Formation are mainly distributed around gypsum pots on the slope of palaeohighs, which shows superior prospect of hydrocarbon potential in this area. Based on core observation, well drilling and logging and 3D seismic data, it can be found that gypsum of Leikoupo Formation has significant geophysical response, including well logging response and seismic response. Gypsum has high values in density curve and natural gamma ray curve. Using the two well logging response, a distinguish plate was made to predict the vertical distribution of gypsum. The predicting coincidence rate can reach 90%. Gypsum shows characteristics of strong aptitude, low frequency and middle-to-low continuity. The seismic amplitude properties is sensitive to gypsum after adjusted by wells. Combined with four amplitude properties selected according to their high coefficient of correlation with wells data, the plane distribution of gypsum was predicted. Compared with the anhydrite map revealed by well data, the prediction result is satisfying. The anhydrite is helpful for the forming of dolomite and dolostone reservoirs, which can be seen from the distribution map of T2l3 anhydrite and dolostone reservoirs in T2l3 and T2l4.
Anhydrite has drawn attentions of petroleum geologists for a long time since it was treated as true geological entities by Schaller and Henderson in 1932. In the recent Moscow 21st World Petroleum Council, the anhydrite related hydrocarbon resource became the hot point for the meeting. Some countries like Angola, Turkey and Norway, even discussed the exploring cooperation area with anhydrite separately. Globally, there are many giant oil and gas fields with anhydrite as cap rocks in North American, Middle East, Siberia. Things are similar in Sichuan Basin southwest China. In the discovered nine large gas fields in Sichuan Basin, only two of them are capped directly by mud rock, the last seven are all covered by anhydrite strata. The hydrocarbon resources in anhydrite related reservoir account for 86.8% of the total proved resources. Zhao et al. (1999) pointed out that in the basins with evaporite rocks, 46% of the basins generate hydrocarbon below the evaporite layers, 41% overlap the evaporite layers, and 13% between the evaporite layers.
