Chemo-sedimentary facies analysis of fine-grained sediment formations: An example from the Lucaogou Fm in the Jimusaer sag, Junggar Basin, NW China

摘要

Fine-grained sedimentary formations occupy about two-thirds of the entire sedimentary sequences in nature. They have recently received great attention owing to the boom of shale oil and gas exploration. Unlike their coarse counterparts, detailed depositional features of fine-grained sedimentary rocks are quite difficult to recognize directly by visual inspection at core or hand specimen scales. A chemo-sedimentary facies analysis (CSFA) procedure has been developed by incorporating high-resolution (down to 30 pm) 2D elemental maps of rock slabs obtained by in situ XRF scanning with high-resolution optical images. The method enables one to unambiguously differentiate rock compositions and fine sedimentary structures and to generate core-scale chemo-sedimentary facies images. The 2D chemo-sedimentary facies map contains major elements with atomic numbers greater than 12, and some trace elements including Ba, V, Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Th and U at individual pixels.