Abstract:

Fine lithofacies research and prediction of marine shale are of great importance for the successful shale gas exploration. Based on the key parameters of shale and fractures, a lithofacies classification scheme for marine shale is proposed by using TOC content and mineral composition. Combined with the TOC, thin section, Xray diffraction and other core testing data, differences of lithofacies for Longmaxi Formation in the Weiyuan area have been analyzed. Furthermore, based on probabilistic neural network (PNN), shale lithofacies of Longmaxi Formation in the Weiyuan area have been predicted by using well log, and the lithofacies associations have been characterized in detail: ① There are 11 lithofacies for Longmaxi Formation in the Weiyuan area, 5 kinds of organic rich shale facies, 6 kinds of organic lean facies. ②Among these 11 lithofacies, the organic rich siliceous shale, organic rich carbonatesiliceous shale and organic rich siliceouscarbonate shale, which contain high TOC content and high brittleness, are favorable lithofacies for shale gas exploration. ③The favorable lithofacies are mainly concentrated on the lowermost part of the Longmaxi Formation, in which thickness increased from northwest to southeast with different favorable lithofacies associations in different part. In the northwest, W1 well has thinner favorable lithofacies associations with thickness of 20 m, mainly composed of organic rich siliceous shale and organic rich carbonatesiliceous shale. W2 well, located in the central part, has different favorable lithofacies associations with thickness of 30 m, mainly consisting of organic rich siliceous shale, organic rich carbonatesiliceous shale, organic rich siliceouscarbonate shale and thin organic rich clay shale. In the southeast, the favorable lithofacies associations of W4 well are mainly composed of organic rich siliceous shale, organic rich carbonatesiliceous shale, organic rich siliceouscarbonate shale and thick organic rich clay shale, which has thickness of 35 m.

Key words: Longmaxi Formation, shale, lithofacies classification, lithofacies prediction, Weiyuan area