The Fuling shale gas field within the Sichuan Basin, Southwestern China, is the first major commercial discovery of shale gas outside of North America in the world, with proven gas reserves of 3,769.85 billion ft3, by the July of 2014. The gas occurs in the lower Paleozoic Wufeng-longmaxi marine shale play, which consists of black shale and has a steady thickness of about 100 meters buried between 2150 and 3150m. The structure of this play is a box-shaped faulted anticline, controlled by two main groups of faults, trending NE and near SN, respectively. For economic considerations, efficient technologies to quickly identify high-quality shale zones within this play are necessary. Hence, integrated geophysical technologies for quantitative predicting total organic carbon (TOC), porosity and brittleness index are developed during this gas field's exploration and development.
Since shale shows a low acoustic impedance (AI) nature, AI inversion combined with post-stack attributes can determine the spatial distribution of shale, which serves as a constraint for follow-up technologies, and also generates shale's porosity volume based on its good correlation to porosity. Through cross-plotting well logs against laboratory TOC logs, density is found to be the most sensitive parameter to TOC within this play; thus, pre-stack density inversion fulfills the goal of quantitative prediction of TOC. Meanwhile, considering the regional geological reasons, two more elastic parameters, µ? and ??, are taken into account to build up a multi-mechanical property formula for predicting brittleness index, while results of using the Rickman formula are not satisfactory in this situation. The application of this new formula has precisely spotted brittle zones within this play. Thereafter, a template, TOC=2%, BI=50%, and porosity=4.1%, for predicting high-quality shale is built up.
The results show the interval, about 30 to 40 meters from the shale formation's bottom, is optimal for horizontal drilling and fracturing, which is proven to be right by later drilled wells. These integrated technologies have greatly reduced drilling risks within this field. It has been proven that they can be applied readily to other shale plays within the Sichuan Basin, which further implies that they can be referenced by other shale plays throughout the world.