Abstract:

Reservoirs of interest in the Da'anzhai Member of the Ziliujing Formation in eastern Sichuan Basin were determined to be interbeds of shell limestone and carbonaceous shale of various thickness based on outcrop observation and thin section authentication with analyses of sedimentary facies, X-diffraction, physical properties, pore structure, and rock mechanics.The assemblage of lake regression limestone interbedded with shale of the sub-lacustrine-slope facies at the upper part of the second sub-member of the Da'anzhai was considered to be the relative good reservoirs as their content of brittle minerals were obviously higher than that of the lower part and were easily cracked and stimulated.Average TOC of the upper part was 1.07% contrasting with a 0.38% at the lower part.Measured total gas concentration of the upper was averaged at 0.71 m³/t, higher than 0.41 m³/t taken from the lower part.Thin section analyses, mercury injection test data as well as physical properties of the samples from the sub-member showed that the reservoirs were extra-low porosity and extra-low permeability.FESEM(field emission scanning electron microscopy)results confirmed the suspect of nano-scale pore structure.Interbedded pores of clay minerals among intergranular pores were considered to have the best connectivity in the sub-member and pores and cracks associated with organic matter as well as perimetral joints of rigid minerals were proved to be well developed at the upper part of the sub-member.The upper part of the sub-member also found to contain some sporadically distributed intragranular pores.It is therefore suggested that the upper part of the sub-member is more promising in terms of exploration potential than any other layers of the Da'anzhai Member.

Key words: pore structure, TOC, shale gas, Da'anzhai Member, Jurassic, eastern Sichuan Basin