Abstract:

This paper first discusses about the geological controlling factors of shale gas plays and various evaluation indicators. It is thought that the evaluation indicators of shale gas can be classified into two types. One type should evaluate the volume of gas, i. e., the resource factors, which decide the resource potential and reserves of this area. This is a key indicator for shale gas resource evaluation and favorable blocks selection, which mainly includes shale thickness, total organic carbon（TOC）, types of kerogen, thermal maturity (Ro), natural gas content and condition (free gas or absorbed gas), etc. The other determines whether the shale gas can be recovered economically as well as the output, i. e., determines a socalled “core zone” or “sweet spot” zone, which includes buried depth, the mineral content of shale itself, brittleness, porosity (fracture), permeability as well as magnitude, orientation and difference of original insitu stress. The two kinds of indicators jointly control the accumulation and final production of shale gas. Therefore, during the process of shale gas exploration and production, the first indicator should firstly be evaluated by utilizing geological and geochemical methods so as to select optimum favorable blocks, while the second indicator can be predicted and evaluated by using seismic related techniques to select optimum shale gas enrichment area or “sweet spot” zone suitable for drilling and production. According to the discussion about geological controlling factors for shale gas enrichment zone (“sweet spot”), the shale gas can be classified into fractured shale gas play and brittle shale gas play. Both of them need to be located in areas of small stress anisotropy so as to obtain complicated fracture network as desired. The author holds that for fractured shale gas play, the geometrical seismic attributes (including coherency, anttracking volume and curvature, etc.) can well characterize fault, subtle fault and fractured zone. The interpretation of these outcomes can be displayed through rose diagram. In this way, the dominant orientation and minor orientation of fractures in this area can be disclosed and then deduce the direction of local principal stress, providing foundation for well deployment. As for brittle shale gas play, prestack inversion and poststack inversion technology can disclose low Poissons ratio, vp/vs ratio and MuRho multicomponent analysis to identify favorable object (“sweet spot”). The seismic technology, once playing a great role in conventional hydrocarbon exploration, is still a necessary technology for unconventional oil and gas exploration and development.

Key words:  shale gas, seismic reservoir prediction, fractured shale gas play