Shale oil and gas are among the most potential and strategic oil and gas replacement resources in China. The second member and third member of the Paleogene Liushagang Formation (Liu-2 Member and Liu-3 Member) in the Weixinan Sag of Beibuwan the Basin in the western South China Sea are the main source rocks, and there are multiple mature laminated shale/sand oil reservoirs. To characterize the quality of shale oil, nuclear magnetic resonance (NMR) and elemental and conventional logging methods are used in this area. These logging evaluation methods are all aimed at the evaluation of total organic carbon (TOC) and open pores of shale oil, which cannot be used to evaluate fluid properties and the formation pressure coefficient of the oil shale.
The wireline formation test (WFT) in shale oil reservoirs will always show tight or dry formation, so the fluid composition, water production, and formation pressure cannot be obtained directly. In the shale/sand laminated reservoirs, the sandstone and shale are in equilibrium, and hence their formation pressure and fluid properties are consistent with each other.
We applied an innovative shale oil evaluation method based on the sandstone WFT data of shale/sand laminated reservoir from the following four aspects. The first aspect was the confirmation of the fluid properties of shale oil laminae by sandstone fluid analysis data. Second was the quantitative permeability evaluation for oil shale based on the integrated analysis of NMR and WFT. The permeability of NMR was calibrated based on the WFT mobility, and then the calibrated NMR data was used to evaluate shale oil and predict matrix productivity of oil shale. This step played an important role in subsequent fracturing design. The third aspect is pressure. The pressure measurement of the sandstone reservoirs revealed that the pressure coefficient of a single well in this area decreases gradually with the increase of vertical depth, and the pressure system between some individual wells is not connected. Meanwhile, the pressure coefficient of shale oil was also obtained, which helped to guide development strategy of shale oil. The fourth aspect is fluid distribution. WFT revealed that the distribution of fluid in each sandstone reservoir interval was different, the gas-oil ratio was 30 to 120 m3/m3, and there were multiple sets of oil-water systems. The formation tests clarified the relationship between the overlying source rock and the oil shale and sandstone and enabled a new understanding of the accumulation model of the sandstone reservoirs, which was confirmed by geochemistry analysis.
Six shale-sand laminated exploration wells were drilled. The shale oil formation in all six wells was evaluated, and three of six wells were chosen to conduct a drillstem test based on the proposed method and achieved an ideal productivity. This new method applies not only to the Weixinan sag but is also applicable to other fields with shale/sand laminated reservoirs.
