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Storage Space Characterization and Its Implications on Lacustrine Shale Oil in China

Abstract

The success of shale gas in North America results in reservoir potential evaluation of shale all over the world. Previous research suggests that pores and fractures are well developed in marine shale systems. China is famous for organic-rich lacustrine shales at oil window, which provide over 80% of total hydrocarbon (HC) sources. Accurate characterization of HC storage space in lacustrine shales is critical to answer whether they can be commercial oil reservoirs. The storage space in shales is composed of both nano-scaled pores and fractures that conventional characterization methods (e.g. visual core examination, petrographic microscopic observation and log analysis) are no longer adequate. Currently, nano- X-ray CT and FIB-SEM are routinely used to reconstruct 3D pore structure models. Functional nano-particles are also employed to improve the accuracy of characterization of nano pores and fractures by injecting them into shales. Other important methods include image analysis, MICP and gas adsorption. Laboratory diagenetic modeling and pyrolysis simulation are used to unravel the porosity evolution and fracture development of shales. A typical ternary compositional scheme is proposed in lacustrine organic-rich shales, including brittle minerals, clay minerals and organic matter. Nano-scaled pores and fractures in dimensions of 50∼300nm are found to be dominant storage space of most oil and gas shales. Inter-clay mineral pores and OM pores account for about 50% and 30% of the total pores, respectively. I/S mixed layers, illite, chlorites are where most pores are discovered in lacustrine shale. In laboratory pyrolysis experiment, OM pores are observed in samples with temperature over 350 centigrade degrees. Moreover, OM porosity generally increases with temperature rising up to 450–475 centigrade degrees and then decreases. Generally, OM porosity in lacustrine shale is smaller than that in marine shale because of maturity difference. The nano pore-throat system in shales is usually interconnected. The above results indicate that lacustrine shale is potential to become oil exploitation targets. However, how to get commercial oil production remains a great challenge for reservoir geologists and engineers. Many issues remain to be addressed including (1) how to evaluate mobile oil in nano-scale pores; (2) how to establish realistic pore structure models for flow simulations; and (3) how to carry out lacustrine shale stimulations for higher production.