四川盆地及周缘龙马溪组热页岩特征及高U值成因

Causes of high U and characteristics of hot shale at Longmaxi Formation in the Sichuan Basin and surrounding areas

与北非地区下志留统热页岩相对应,在四川盆地及周缘龙马溪组底部同样发育了一套大面积分布、高伽马值、高放射性的热页岩,综合利用钻井、测录井以及地化等资料,在建立该套热页岩的识别标准的基础上,开展了热页岩发育特征、高U值成因的研究,并探讨了该套热页岩在页岩气勘探中的意义。结论认为:第一,区别于北非地区利用GR值划分热页岩,四川盆地及周缘利用页岩的密度和铀值能更好的将龙马溪组热页岩(TOC≥2%)识别出来,通常热页岩的密度小于2.62 g/cm^3,铀值大于8.5×10^(-6);第二,发现四川盆地及周缘龙马溪组热页岩总体发育,在远离物源的深水陆棚区厚度一般大于20 m,但在湘鄂西来凤—宣恩—恩施地区发生异常,明显变薄,一般在1.5~10 m,分析宜昌上升是造成该地区热页岩不发育的主要原因;第三,热页岩中碳酸盐矿物含量在平面分布上有所差异,在川南—川西南地区含量普遍大于10%,为含钙质硅质热页岩,而在川东北—湘鄂西—川东南则普遍小于10%,主要为硅质热页岩;碳酸盐矿物含量的差异会影响热页岩的可压性并造成压裂液配方不同;第四,在龙马溪组热页岩底部发现了一套"异常高铀、异常高伽马"的层段,整套热页岩不仅对页岩气富集、高产起到积极的作用,同时其还可作为水平井钻进的标志层,对水平井在优质热页岩高中靶具有良好的指示作用。

Corresponding to the hydrothermal shale of lower Silurian in North Africa, at the bottom of Sichuan Basin and peripheral edge Longmaxi formation also widespread distributed a large area of highly gamma and highly radioactive hydrothermal shale, the article used comprehensive utilization of drilling, logging, geochemical and other information, on the basis of the establishment of identification standards; we studied thermal characteristics of shale and genesis of high U value, and also its significance in shale gas exploration. Then concluded that： Firstly, different from classified by GR values in North Africa, Sichuan Basin and peripheral edge used density and the uranium values which can better identify Longmaxi formation hydrothermal shale （TOC≥2 %）,and density of hydrothermal shale generally is less than 2.62 g/cm3and uranium values is greater than 8.5× 10-6. Secondly, we found that hydrothermal shale was overall developed in Sichuan Basin and peripheral edge Longmaxi formation, and in deep water continental shelf region which away from provenance ,the thickness is generally greater than 20 meters, but exception occurred in the Western Hunan and Hubei Lai feng-Xuan＇en-Enshi, the thickness became obviously thinner, we analysis that the rise in Yichang is the main cause of hydrothermal shale undeveloped; Thirdly, the content of carbonate minerals in hydrothermal shale distributed differently in plane in southern and southwest Sichuan, mainly the siliceous calcareous hydrothermal shale which generally greater than 10 %, and in northeastern Sichuan-Western Hunan and Hubei-Southeast Sichuan were generally less than 10 %, mainly siliceous shale heat; the carbonate mineral content will affect the thermal compressibility and cause different fracturing fluid. Finally, in the bottom of the Longmaxi formation hydrothermal shale we found a set of ＂unusually high uranium, unusually high gamma＂ formation, this shale not only play a positive role in gas accumulation and high production, but also can be a marker bed of horizontal well which indicate to shoot the target.