鄂尔多斯盆地东部下奥陶统碳酸盐──蒸发盐地球化学特征和生气规律

Geochemical Characteristics and Gas-producing Rules from Carbonate-Evaporite Formation of Lower Ordovician Series in East Ordos Basin

本文用有机地球化学和有机岩石学方法，研究了鄂尔多斯盆地东部下奥陶统马家沟组碳酸盐—蒸发盐沉积的钻孔地球化学剖面。论述了奥陶系马家沟地层存在Ａ．Ｂ．Ｃ３个非干旱条件的水下蒸发盐沉积旋回，对有机质的聚集和保存都十分有利。源岩以菌藻类水生生物输入为主，并具有高盐和强还原环境。源岩的有机碳含量在０．０８％—０．２０％左右，最高可达０．６４％。烃源岩的热演化程度较高，镜质体反射率Ｒ０已达１．５—２．０％左右，但在一些样品中也发现有荧光沥青、固体沥青和有机包裹体等烃类生成、赋存的重要信息，表明本区马家沟组碳酸盐—蒸发盐是较好的气原岩。

In this paper, we present the systematic geochemical sections of carbonate - evaporite sediments from Majlagou Formation,Lower Ordovician series in east Ordos Basin by means of organic geochemistry and organic petrology. It demonstrates that the conditions of three sub aquatic evaporite sedimentary cycles,which formed in a non - dry climate in Majiagou Formation, are favourable for accumulation and preservation of organic matters. The source rocks contain low organic carbon (TOC= 0. 08￣0. 20 %, maximum up to 0. 64 % ). Characteristics of biomarker assemblage include: low Pr/Ph ratios (0. 6￣0. 8±), widespread of β-carotane. pregnane and homo - pregnane. high tricyclic terpane/pentacyclic terpane ratios (1. 5±), low abundance of gammacerane in some samples, high ahundant dibenzothiophen and lack of dibenzofuran in aromatic hydrocarbons. These biomarker characteristics reflect that sedimentary organic matters are dominantly derived from algae and/or bacterias, and the depositional environment is hyper saline and highly anoxic.Based on the results of vitrinite reflectance, bitumen reflectance and inclusion homogenerization temperatures, it can he seed that the Majiagou Formation source rocks have undergone two large cycles of thermal evolution. The first cycle of thermal evolution is weak, while the second is violent and mainly controlls the generation and productive capacity of natural gases from Lower Palaeozoic Ordovician system and Upper Palaeozoic Carboniferous- Permian system. Though the Ordovician source rocks are now up to high maturation degrees (vitrinite reflectance values up to 1. 5￣ 2. 0 % ), some information of hydrocarbon generation has been discovered, such as solid bitumen, fluorescent asphalt and organic inclusions. etc.. These features illustrate that carbonate-evaporite formation is a good source rock.