Sedimentary characteristics and gas accumulation potential of the Ordovician carbonate-evaporite paragenesis system in central and eastern Ordos Basin

doi:10.13745/j.esf.sf.2022.8.25

Abstract

Abstract:

A sedimentary system consisting of interactive ultra-thick carbonate rock and gypsum-salt rock developed in the Ordovician Majiagou Formation in the central and eastern Ordos Basin, however, its development process and natural gas accumulation characteristics are not clear. By studying its tectonic and paleogeographic background, sedimentary features, reservoir development, hydrocarbon source supply and trap accumulation, we gain new geological insights into the carbonate-evaporite paragenesis system, which can be summarized as follows: 1) Affected by its surrounding ancient uplifts, the carbonate-evaporite paragenesis system is formed in a restricted sea with periodic isolation from the open sea, therefore its lithologic distribution has obvious “facies-control” and “strata-bound” characteristics. 2) The key factor controlling the development of the Ordovician “interior dissolved pore” type reservoir is “nonstratiform sedimentary differentiation”—a novel concept, proposed here for the first time. 3) Controlled by regional lithologic transition and later structural inversion, the Ordovician sub-salt stratum has favorable conditions for forming lithologic trap system on a large scale. 4) Although the sub-salt stratum has limited hydrocarbon-generation capacity from its own marine source rocks, it still has the potential of large-scale hydrocarbon supply/accumulation due jointly to the Yanshanian tectonic uplift in its eastern part, hydrocarbon supply from Upper-Paleozoic coal-bearing source-rock on its west side, and regional continuous distribution of gypsum-salt capping strata.

Key words: Ordos, Ordovician, nonstratiform sedimentary differentiation, carbonate-gypsum rock, sub-salt hydrocarbon accumulation, window for hydrocarbon supply

CLC Number:

P618.13

BAO Hongping, WANG Qianping, YAN Wei, CAI Zhenghong, ZHENG Jie, WEI Liubin, HUANG Zhengliang, GUO Wei. Sedimentary characteristics and gas accumulation potential of the Ordovician carbonate-evaporite paragenesis system in central and eastern Ordos Basin[J]. Earth Science Frontiers, 2023, 30(1): 30-44.

Figures/Tables 13

Fig.1 Structural and sedimentary evolution profile of Ordovician in Ordos Basin. Modified after [12].

Fig.2 The distribution map on paleouplift and salt depression of Ordovician in Ordos area

Fig.3 Lithofacies paleogeography map of Ordovician Ma.3 period(a) and Ma.4 period (b)in Ordos area

Fig.4 Sedimentary model and lithofacies profile of Ordovician Majiagou period in Ordos area

Fig.5 Rock structural characteristics of layered and non layered differentiation of Ordovician carbonate and gypsum-salt system in central-eastern Ordos Basin

Fig.6 Reservoir micro pore characteristics of Ordovician carbonate gypsum-salt rock system in central-eastern Ordos Basin

Fig.7 The distribution diagram of gypsum-salt rock in Ma 6 sub-member of Ordovician in Ordos Basin

Fig.8 Hydrocarbon source rock structure and organic matter distribution characteristics of Ordovician carbonate gypsum-salt rock system in Ordos Basin

Table 1 Statistics of organic matter abundance of Upper Paleozoic coal measure source rocks in Ordos Basin

地层	本溪组		太原组		山西组
地层	厚度/m	TOC含量/%	厚度/m	TOC含量/%	厚度/m	TOC含量/%
煤层	7~20	55~80	4~8	10~75	5~15	45~90
暗色泥岩(碳质泥岩)	30~40	3~20	20~30	2~18	50~70	1~15
灰岩	4~12	0.2~1.2	10~30	0.3~1.5

Fig.9 Unconformity contact between Ordovician and Upper Paleozoic on the east side of Central Paleo-uplift

Fig.10 The tectonic changes after Ordovician sedimentary period in Ordos aera

Fig.11 Gas accumulation model of Ordovician carbonate gypsum-salt rock system in central-eastern Ordos Basin

Fig.12 Cretaceous paleostructure map and Distribution prediction of favorable undersalt hydrocarbon accumulation zones in Ordovician

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