湖北宜昌地区鄂阳页2井牛蹄塘组和陡山沱组页岩气随钻碳同位素特征及勘探意义

为了深入分析页岩气的甜点特征，针对鄂阳页2井所钻遇的牛蹄塘组和陡山沱组两套古老页岩层系，在井场随钻连续取样，并测量泥浆气碳同位素值以及岩屑罐顶气碳同位素随时间序列的变化特征。基于高密度连续碳同位素数据，结合气体组分、地层岩性、储层物性特征分析页岩气的成因及富集规律；基于岩屑释放气碳同位素分馏特征，结合页岩纳米孔隙发育程度分析页岩气地质甜点要素。研究结果显示：甲烷和乙烷倒转特征及湿度特征表明牛蹄塘组页岩可能具有较好的页岩气勘探潜力；泥浆气甲烷碳同位素值随地层垂深上的变化与生油窗时期液态烃类的残留有关，牛蹄塘组页岩底部同位素值偏轻揭示其液态烃类残留量大，资源潜力高；陡山沱组页岩气组分偏干、碳同位素值偏重的特征可能与其生烃母质以及生、排烃演化史有关；岩屑释放气同位素分馏特征与页岩纳米孔喉体系的发育程度有关，纳米孔喉体系发育程度作为页岩气甜点的判断指标应是一条可行的研究思路。这种基于现场的、连续的、动态的碳同位素分析方法为页岩气成因及富集规律分析、地质甜点区判识提供了更多有价值的参考。

Characteristics of carbon isotope while drilling and exploration significance of shale gas in Niutitang and Doushantuo formations in Well Eyangye-2, Yichang,Hubei, China

To further analyze the characteristics of sweet spot of shale gas, aiming at two sets of ancient shale series of strata in Niutitang and Doushantuo formations drilled in Well Eyangye-2, continuous sampling while drilling was conducted in the well site; the changes of the carbon isotope values of mud gas and the carbon isotope values of cuttings head space gas with time series were measured. Based on the continuous high-density carbon isotope data, the genesis and enrichment rule of shale gas were analyzed in combination with gas composition, formation lithology and reservoir physical properties. Based on carbon isotope fractionation characteristics of the gas released from cuttings, the geologic elements of sweet spot in shale gas reservoirs were analyzed in combination with the development degree of nanopore in shale. The results show that the reversal characteristics and humidity characteristics of methane and ethane indicate that Niutitang Formation may have good potential for shale gas exploration. The variation of carbon isotope value of mud gas with true vertical depth (TVD) is related to the residual liquid hydrocarbons in the oil generation window. The light isotope at the bottom of the Niutitang Formation shale reveals that there are large quantities of residual liquid hydrocarbons, showing great resource potential. The dry component and heavy carbon isotope of the Doushantuo Formation shale gas may have certain relation with the hydrocarbon generation material and the evolution history of hydrocarbon generation and expulsion. The isotope fractionation characteristics of the gas released from cuttings are relevant with the development degree of nano pore-throat system in shale. It should be practical to use the development degree of nano pore-throat system as the judgment index of sweet spot in shale gas. This field-based, continuous, and dynamic analysis method of carbon isotope provides more valuable references for analyzing the genesis and enrichment law of shale gas and identifying geological sweet spots.