海相页岩有机质碳化的电性证据及其地质意义——以四川盆地南部地区下寒武统筇竹寺组页岩为例

寻找一种能够有效表征海相页岩有机质碳化现象的可靠证据,是当前开展古老地层页岩气选区面临的重大科学问题。为此,以钻井岩心、测井和分析化验资料为基础,通过对比分析四川盆地南部地区下寒武统筇竹寺组、下志留统龙马溪组2套富有机质页岩的电性特征,探索研究了筇竹寺组页岩有机质碳化现象及其对页岩气赋存条件的影响。获得以下认识:1电性实验证实,川南地区筇竹寺组富含有机质页岩已出现有机质碳化特征,根据极高成熟度、富有机质页岩的干岩样低电阻率特征和井下超低电阻率测井响应特征,能够直观判断有机质的碳化程度、评价烃源岩质量;2川南南部长宁地区筇竹寺组页岩表现为干样低电阻率和低—超低测井响应特征,有机质已出现严重碳化现象,威远地区筇竹寺组页岩电阻率曲线基本正常,石墨化程度介于龙马溪组和长宁筇竹寺组之间;3中高电阻率页岩在生气潜力、基质孔隙发育程度和对天然气吸附能力等方面明显优于超低电阻率页岩;4川南北部犍为—威远—潼南地区富有机质页岩具有中—高电阻率特征,钻井已获工业气流,为筇竹寺组页岩气勘探开发的有利区。

Electric property evidences of the carbonification of organic matters in marine shales and its geologic significance:A case of the Lower Cambrian Qiongzhusi Shale in southern Sichuan Basin

Searching for some reliable evidences that can verify the carbonification of organic matters in marine shales is a major scientific issue of facing play fairway identification in old shale gas plays. Based on core, logging and testing data, an electric property correlation was performed between the two organic rich shale layers in the Lower Cambrian Qiongzhusi Fm and the Lower Silurian Longmaxi Fm in southern Sichuan Basin. The results were used to analyze the carbonification of organic matters in the Qiongzhusi Shale and its influence on gas storage in the shales. The following conclusions were obtained. (1) The electric properties verify the carbonification of organic matters in the Qiongzhusi Shale in the study area. The dry samples from this highly mature organic rich shale show a low resistivity, while showing an ultra low resistivity on downhole logs. These resistivity values can be used to directly determine the degree of organic matter carbonification and the quality of source rocks. (2) In the Changning area, the Qiongzhusi Shale shows a low resistivity of dry samples and low to ultra low resistivity on logs and the organic matters are seriously carbonized, while in the Weiyuan area, the Qiongzhusi Shale shows a basically normal resistivity and its degree of graphitization is between the Longmaxi and Qiongzhusi Fms in the Changning area. (3) The shale with a medium to high resistivity is remarkably better than that with an ultra low resistivity in the respects of gas generation potential, matrix porosity and gas adsorption capacity. (4) Industrial gas flow was tested in the organic shales with a medium to high resistivity in the Jianwei Weiyuan Tongnan area in the north, where the Qiongzhusi Shale is a favorable shale gas exploration target.