Quantitative determination of the brittle-ductile transition characteristics of caprocks and its geological significance in the Kuqa depression, Tarim Basin, western China

The Tarim Basin is one of the most hydrocarbon-bearing basins in China. It has two sets of gypsum and salt regional caprocks, which have various thicknesses and areal distributions. Such caprocks control large-scale hydrocarbon accumulation and the reservoirs formation in the basin. The estimation of the brittle-ductile transition characteristics of caprocks is one of the fundamental geological problems in petroleum exploration. To tackle this problem, a preliminary experimental method based on the mechanical characteristics of rocks is presented in this paper, which quantitatively forecasts the brittle-ductile transition characteristics of caprocks. The experimental study is based on triaxial tests on different gypsum and mudstones from caprocks of Jidike and Kumugeliemu Formations sheared at different confining stresses. It is shown that during burial, gypsums behave as normally consolidated materials and exhibit ductile responses to load increase. With the increase of depth, gypsum transforms from brittle stage to brittle-ductile or ductile stage. The confining pressure for brittle to brittle-ductile (semibrittle) transition of gypsum is about 18 MPa, and that of brittle-ductile to ductile transition is about 62 MPa. By analyzing the internal structure of fault zones from the caprocks in different brittle-ductile stages, it was observed that faults in brittle gypsum rock deform and form through-going faults, which are filled with soft fault gouge and often act as vertical migration pathways for hydrocarbon. Faults deformed in brittle-ductile caprocks form smears. A shale smear factor (SSF) of 3.5 is the critical value to discriminate between continuous and discontinuous gypsum smears, which can serve as a standard for determining the vertical sealing of the faults in brittle-ductile caprocks. It is generally difficult to cm through faults in ductile gypsum caprocks; this is significant to the sealing of the hydrocarbon vertical migration pathways.