海相镜质体反射率用作早古生代烃源岩成熟度指标研究

Marine vitrinite reflectance as a maturity indicator of lower Palaeozoic hydrocarbon source rocks

中国早古生代烃源岩由于缺乏镜质组，加之成熟度较高，常现成熟度指标较难应用，长期以来，对这套烃源岩成熟度评价一直存在问题。塔里木盆地是中国最大的以早古生代地层为主的含油气盆地，在寒武系和奥陶系生油岩中发育有海相镜质体。应用有机岩石学方法主要对塔里木盆地四口深井岩芯样品进行了研究，发现海相镜质体反射率与孔深呈正相关，且其相关性优于沥青反射率，人工热模拟实验结果进一步表明，海相镜质体成熟作用轨迹不同于镜质组。在R＜1．5％时，其反射率比镜质组低；在R＞1．5％时，其反射率比镜质组高。结合天然剖面与热模拟系列样品研究成果，发现R与Ro的相关性可划分为三个阶段，可分别近似用三个直线方程表示：根据海相镜质体反射率实测结果，应用这些方程计算出等价Ro，可较客观地评价早古生代烃源岩成熟度。

Because of the absence of vitrinite and higher maturity of the lower Palaeozoic sediments inChina, the evaluation of their maturity has heen a matter not to be solved perfectly. The TarimBasin is one of the largest petroliferous basins in China where the lower Palaeozoic marinesediments are dominant and a type of marine vitrinite occurs widely in these marine strata. Atotal of 35 lower Palaeozoic source rock samples mainly from four boreholes in the basin havebeen investigated by organic petrologic methods and a positive correlation between marinevitrinite reflectance and depth of samples has heen recognized. This correlation is better thanthat between hitumen reflectance and depth. Artificial maturation data through which thematuration pathway of marine vitrinite is shown is quite different from that of vitrinite. Thereflectance of marine vitrinite shows jump development with increasing maturation. Based onthe combination of reflectance data of naturally and artificially matured samPles, therelationship between marine vitrinite reflectance and vitrinite reflectance is divided into threestages which can be represented largely by three linear regression equations, respectively. Theyare: Ro=1. 26 R+0.21 (R<0.75 % ); Ro =0. 28 R+1. 03 (R:0. 75 ～ 1. 50 % );Ro = 0. 81 R + 0.18 (R>1. 50). These equations might provide petroleum geologists witha convenient method to evaluate the maturity of lower Palaeozoic sediments by applying theequations to obtain an equivalent vitrinite reflectance according to measured marine vitrinitereflectance.