Pore-structure And Petrophysical Properties Of Silt Laminae In Lacustrine Gas Shale: Example From The Yanchang Group, Ordos Basin

The proved success in shale gas exploration and production in North America hasattracted the eyes of China which has become the biggest resource consuming country of theworld. No one knows whether the legend could be repeated in China since the source rockthere is distinguished by its lacustrine origin. However, the recent discovery of gas loggingabnormal in the shale sequence of the upper Triassic Yanchang Formation, Ordos Basin nodoubt gain the explorers’confidences in continental shale gas. It has been studied that the finesediments of semi-deep and deep lake facies in Chang7member and Chang9member,named as the Zhangjiatan Shale and the Lijiapan Shale are source rocks of high quality withsome other favorable conditions for shale gas production, say relatively shallow buried andlarge area. Therefore, the petrophysical properties of the shale become the first considerationfor reserve estimation and stimulation design.To determine the porosity and permeability of the shale, its intrinsic pore structure is thekey factor, which to some extent is controled by the lithology. Considering the frequentlithofacies changes in lake sediments, it is not surprized to find numerous silt laminaeinterbeded with shale in the core and crop observation in the study area yet always overlookedby the well logs. The silt laminae add heterogeneity to the whole shale gas reservoir and it hasbeen reported that they are important to gas production in the none-fractured marine shaleintervals. Anyway, the role of silt lamnae in shale is ambigous. They are supposed to be moreporous and permeable than the clay-rich laminae yet no detailed petrophysical data or microstructure understanding about silt laminae available by now.This paper sheds light on the differences between the silt laminae and clay laminaedeveloped in the potential gas shale in the study area from the view of pore structure andpetrophysics. By separating the silt laminae from the shale core mechanically, a series of selected lab testing are conducted carefully both on the two parts including mercury injection,gas adsorption and SEM observation. The pore structure of mixed core samples with matchedXRD component analysis is also studied in the same way as a supplement. In addition, thewhole core analysis like He-porosimetry and PDP-permeability are also done on samples withdifferent silt laminae pecentage. The results show that the silt laminae develope micron-sizedintergranular pores and solvopores that are much larger than the nano-sized phyllosilicateframework ones abundant in shale. The meso-and macropore volume of the silt laminae isalso bigger than that of the shale and contributes the most compared to the micropore volume.Correspondingly, the tested porosity and permaeability are both higher in the shale samplescontaining larger proportion of silt laminae. Therefore, the silt laminae in shale are preferredcandidates for gas storage and adquate conduits for fluid flow.