川东北飞仙关组-长兴组天然气几个地球化学问题探讨

Discussion on several geochemical issues of gases in Feixianguan-Changxing Formations in northeastern Sichuan Basin,China

基于川东北地区大普光、元坝和通南巴构造带百余个飞仙关组-长兴组天然气样品的分子和碳同位素组成数据,结合烃源岩和储集岩分析资料,就原油裂解气与烃源岩裂解气的区分、烷烃气碳同位素的反序分布和CO_2与H_2S的成因关系等问题进行了探讨。研究结果表明,大普光、元坝区块富含固体沥青的孔洞型气藏的原油裂解气中,丙烷相对较多,以较低的ln(C_2/C_3)值(<3.0)为识别标志。而通南巴等区块裂缝型气藏的烃源岩裂解气(可溶沥青和干酪根的高温裂解气),具有ln(C_1/C_2)和ln(C_2/C_3)同步升高的组成特征,以较高的ln(C_2/C_3)值(>3.0)与典型的古油藏原油裂解气相区别。各构造带的飞仙关组-长兴组烷烃气存在碳同位素反序分布,可能有多种原因。其中,通南巴构造带河坝场气田飞仙关组烷烃气中的该现象,是由于龙潭组过成熟干气混入志留系气源气所致。飞仙关组-长兴组发生过TSR(硫酸盐热化学还原反应)作用的天然气中,多呈高CO_2、低H_2S和低CO_2、高H_2S两种分布模式,两种非烃气的相对含量受气藏流体-岩石相互作用体系的控制。在高含H2S的气藏中,CO_2主要来源于烃类的氧化,并经流体-岩石交换作用,其δ^(13)C值相对较负;而在CO_2异常丰富的天然气中,CO_2主要由碳酸盐岩的化学分解而来,δ^(13)C较重。

Molecular and isotopic composition data of over hundred gas samples were combined with analytical data of source and reservoir rocks to deal with geochemical uncertainties of gases in the Feixianguan-Changxing Formations in Puguang,Yuanba and Tongnanba regions in northeastern Siehuan Basin. Discussion was focused on differentiation of cracking gases from oil and source rocks, causation of carbon isotope reversals in hydrocarbon gases and genetic relation- ship between CO2 and H2S. The results show that oil cracking gases from vuggy reservoirs abundant in solid bitumen in Puguang and Yuanba regions are relatively enriched in propane and have low ln（C2/C3） value （〈3.0） as a diagnostic signature. In contrast,the source rock cracking gases （sourced from kerogen and bitumen cracking） from fractured reser- voirs in Tongnanba region, exhibit a co-varying In （ C1/C2 ） and In （ C2/C3 ）, and display higher values （ 〉 3.0） of ln（ C2/C3 ）, distinguishing from typical oil cracking gas. Isotope reversals, resulted possibly from various factors, appear extensively in hydrocarbon gases from the Formations. For the paraffin gas from the Feixianguan Fm of Hebachang gas field in Tongnanba region, the isotope reversal can be attributed to a mixture of post-mature dry gas generated from the Upper Permian Longtan source rocks and gas with slightly higher wetness coming from the Silurian source. Two non- hydrocarbon gas composition modes,namely high H2S content-low CO2 content and low H2S content-high CO2 content,are recognized in the gases experienced TSR from the Feixianguan-Changxing reservoirs. The relative proportions of these two non-hydrocarbon gases may be associated with overall interaction system of reservoir fluids and rocks. CO2 with somewhat negative carbon isotope is possibly resulted from hydrocarbon oxidizing in high H2S reservoirs, whereas, isotopically heavier C（）2 couh] be mainly formed by chemical degradation of carbonate when it presents in abnormally high amount.