Shale Reservoir Characteristics of Lower Silurian Longmaxi Formation in Southeast Chongqing, China
He Bi1,2, Peng Li1,2, Zhenxue Jiang1,2, Yuan Yuan1,2, and Ye Xu1,2
1State Key Laboratory of Petroleum Resources and Prospecting, CUPB, China
2Unconventional Natural Gas Institute, CUPB, China
Southeast Chongqing is located on the edge of southeast Sichuan Basin, which belongs to the central area of shale gas exploration and development in the Yangtze region where widely developed Longmaxi Formation organic-rich shale. The anticlines and faults in this area are Caledonian structural layers of Yanshan fold, mainly forming in Mesozoic Yanshanian period. This research selected the Longmaxi Formation shale cores to do thin section analysis, X-ray diffraction (XRD), nitrogen adsorption test (NAT) and scanning electron microscopy (SEM) to study the petrologic feature, mineral feature and microscopic pore feature, through which the occurrence state of shale gas can be better understood to assist evaluating resource potential and guiding the exploration of shale gas.
Thin section analysis reveals that many lithologies are identified in Longmaxi Formation shale like clay- to silt-sized fine-grained sediments, such as silty carbonaceous mudstone, silty argillaceous mudstone, argillaceous mudstone and argillaceous siltstone. Most of shale is organic-rich with abundant graptolite fossil and pyrite, which represents strong reducing environment. Under this palaeoenvironment with more saline water, it was conducive for organic matter to preservate. And now the average of total organic carbon (TOC) reaches to 2.50%. Silty shale and argillaceous siltstone developed laminated structure, partial of which was slightly destroyed by faint biological disturbance and then resulted in mottled structure. Some of chlorite and particulate organic matter indicate the stratification.
XRF results exhibit that mineral component of Longmaxi Formation shale mainly comprises detrital minerals and clay minerals, secondly comprising a small quantity of carbonate minerals and pyrite. Detrital minerals content ranges from 42% to 71.5%, with an average of 61.4%. Quartz averagely occupies 49.5%, which is also mainly component of detrital mineral. The left are feldspar and mica. Clay minerals content ranges from 12.1% to 44%, averaging of 25.2%. Illite and illite-montmorillonite mixed-layer are the primary clay minerals, among of which these two minerals respectively own average relative content of 49.1% and 41.8%. The few left is chlorite. Under the microscope, it is discovered that most of quartz is distributed unevenly in the shale and a small amount of it with feldspar shows weakly direction arrangement. The clay minerals, such as illite, mix with a small amount of fine grained kerogen and scatteredly distributed pyrite. Also it is easy to find that distribution of chlorite is parallel to bedding plane. The siliceous sediments of this shale mostly came from siliceous organism. As the silica precipitating, organism remains gradually increased, which raised the content of organic matter. Accordingly, the adsorption capacity of shale became stronger. In addition, illite and illite-montmorillonite mixed-layer also offer the adsorption space for shale gas.
Thermal maturity (RO) of Longmaxi Formation shale averagely reaches to 2.35%. During the thermal evolution process, organic matter thermal cracking produced abundant organic nano-scale pore (d<0.75µm). In the light of NAT analysis, which offers a consistent methodology for studying the organic pore structure in shale, Longmaxi Formation shale exhibits the organic nano-scale pore owning total volume of 5.57×10-3 to 2.51×10-3ml/g and average volume of 1.67×10-3ml/g. Among nanoscale pore, mesopore (d=2~50nm) occupies 64.7% of total volume, however micropore (d<2nm) and macropore (d>50nm) respectively occupy 12.3% and 23.0%. Rich organic nano-scale pore provides larger pore specific surface area which is avail to shale gas adsorption. Studied from a series of SEM images, micro-scale pore (d>0.75μm) is likewise various, including quartz intergranular pore, feldspar dissolved pore, clay mineral intercrystal pore, pyrite intergranular pore, etc. These kinds of pore were developed mainly due to organic acid dissolution and transformation between minerals. In addition, tectonism resulted in microfracture which assisted improving the permeability of the shale reservoir. Statistics reveal that porosity of the shale ranges from 0.47% to 5.61%, with an average of 2.42% and permeability is generally less than 0.01mD. Thus, it is beneficial to accumulate shale gas in free form.
In conclusion, Longmaxi Formation shale in southeast Chongqing mainly develops clay- to silt-sized fine-grained sediments, most of which are organic-rich shale, with abundant graptolite fossil. Mineral component mainly comprises detrital minerals (like quartz) and clay minerals (like illiate), secondly containing pyrite. Moreover, many kinds of micro-scale pore develop, including quartz intergranular pore, feldspar dissolved pore, clay mineral intercrystal pore, pyrite intergranular pore, etc. Because of the organic matter thermal cracking, abundant nano-scale pore is developed in the shale and mesopore occupies the most. Consequently, the shale reservoir condition of Longmaxi Formation in southeast Chongqing is beneficial for shale gas occurrence in adsorbed and in free forms, which has positive response to further exploration and development for shale gas.
AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013