Discovery of syngenetic and eogenetic karsts in the Middle Ordovician gypsum-bearing dolomites of the eastern Ordos Basin (central China) and their heterogeneous impact on reservoir quality
Discovery of syngenetic and eogenetic karsts in the Middle Ordovician gypsum-bearing dolomites of the eastern Ordos Basin (central China) and their heterogeneous impact on reservoir quality
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鄂尔多斯盆地东部中奥陶世含膏白云岩中同生和早生岩溶的发现及其对储层质量的非均质影响
DOI：
10.1016/j.marpetgeo.2018.10.004
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发表时间：
2019
影响因子：
4.2
通讯作者：
Cao Jian
中科院分区：
文献类型：
--
作者：
Xiao Di;Tan Xiucheng;Zhang Daofeng;He Wei;Li Ling;Shi Yunhe;Chen Juanping;Cao Jian
鄂尔多斯盆地东部(中国中部)中奥陶统马家沟组是典型的含膏碳酸盐岩层岩溶实例。它在沉积至晚石炭世约120 Ma的漫长岩溶历史中是独一无二的。岩性复杂，发育细粒白云岩和粒状白云岩。因此，岩溶作用及其对水库质量的影响是具有挑战性的。为了加深对这一关键问题的认识，进一步为勘探提供资料，我们采集了具有代表性的马五段第一亚段和第二亚段(本文简称马51+2)的岩心样品，并对其物理和地球化学性质进行了综合分析。结果表明，Ma 51+2一般由含石膏的泥晶白云岩和颗粒白云岩组成。观察到了岩溶角砾岩，并识别出两期岩溶作用。第一期为同生期，表现为石膏型、粒内孔隙等典型特征。由于选择性的组构溶解，它主要发育在含石膏的泥晶白云岩中。同位素数据显示了同生大气水和海水的影响，表现为δ13C值略有亏损(平均−0.03‰)，δ18O值相对较低(平均−8.35‰)，87Sr/86Sr值略有升高(平均0.70999)。第二期为内陆成岩岩溶作用，具有典型的非选择性组构溶解特征，包括颗粒岩中的孔洞、角砾岩和岩溶充填物。岩石经历了差异脱白云岩化作用。大气降水强烈地改变了颗粒白云岩，并记录了陆地输入，负碳和氧同位素漂移(δ13C和δ18O平均值分别为−3.07‰和−8.97‰)和高的87Sr/86Sr值(平均0.71093)表明。两期岩溶作用对储集层质量的影响是深刻的、非均质的。含膏结核泥晶白云岩经早期同生岩溶作用改造形成的石膏型，代表了Ma 51+2的最佳储集空间，反映了同生岩溶作用在优质储集层形成中的关键作用。相比之下，内陆晚期成岩作用主要表现为颗粒白云岩强烈蚀变，泥晶白云岩物性较差，对泥晶白云岩影响较小。强烈的溶蚀作用导致的坍塌破坏了储层的连续性。此外，相对较高的孔隙度和渗透率促进了流体的运动，导致泥质和碳酸盐砂体充填了岩溶洞穴系统。因此，发生了所谓的物性反转，即颗粒白云岩中原始大孔隙的充填最终导致储集性能变差，而泥晶白云岩中原始小孔隙的保存最终导致储集性能相对较好。这说明在岩溶作用的影响下，储集层的演化是复杂的。
The Middle Ordovician Majiagou Formation in the eastern Ordos Basin (central China) represents a typical case of karst in gypsum-bearing carbonate strata. It is unique in a long karst history of ca. 120 Ma since the deposition to the Late Carboniferous. The lithology is complex with the development of both fine-grained and granular dolomites. Thus, the karstification and its impact on reservoir quality is challenging. To improve the understanding of this critical issue and further provide data for exploration, we collected core samples within the representative first and second sub-members of the fifth Member of the Majiagou Formation (herein referred to as Ma 51+2) and conducted comprehensive analyses of their physical and geochemical properties. Results show that Ma 51+2is generally composed of gypsum-bearing micritic dolomites and grain dolomites. Karst breccias were observed and two periods of karstification were recognized. The first period is syngenetic and shows typical features such as gypsum molds and intragranular pores. It is predominantly developed within gypsum-bearing micritic dolomites due to selective fabric dissolution. Isotopic data indicate the influence of syngenetic meteoric water and seawater, as indicated by slightly depleted δ13C values (average −0.03‰), relatively low δ18O values (average −8.35‰), and slightly elevated87Sr/86Sr values (average 0.70999). The second period is interpreted as inland eogenetic karstification and shows typical features such as non-selective fabric dissolution, including pores and vugs, breccias, and karst fillings in grain rocks. The rocks were subjected to differential dedolomitization. Grain dolomites were strongly altered by meteoric water and record a terrestrial input, as suggested by negative carbon and oxygen isotope excursions (average δ13C and δ18O values are respectively −3.07‰ and −8.97‰) as well as high87Sr/86Sr values (average 0.71093). The two periods of karstification had a profound and heterogeneous influence on the reservoir quality. Gypsum molds formed due to the alteration of gypsum-nodule-bearing micritic dolomite by early syngenetic karstification, and represent the best reservoir space in Ma 51+2. This reflects the key role of syngenetic karstification in the formation of high-quality reservoirs. In contrast, the late inland eogenetic karstification is mainly manifested as strong alteration of grain dolomites, with little effect on micritic dolomites due to its relatively poor physical properties. The collapse resulting from strong dissolution destroys the continuity of reservoirs. In addition, relatively high porosity and permeability promote fluid movement, resulting in the infilling of the karst cave system by mud and carbonate sand. Thus, a so-called inversion of physical property occurs, namely the filling of original large pores in the grain dolomites leads to poor reservoir at last, and the preservation of initially small pores in the micritic dolomites leads to relatively good reservoir at last. This indicates that the evolution of reservoir is complex under the influence of karst.
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影响因子：
1.8
作者：
Ma Zhanrong
通讯作者：
Ma Zhanrong
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