Fracture pore characteristics and gas accumulation model of marine shales in the northwestern Ordos Basin: A case study of the Ordovician Wulalike Formation

doi:10.13745/j.esf.sf.2023.6.14

Abstract

Abstract:

The Ordovician Wulalike Formation in the northwestern margin of the Ordos Basin is a new prospect for exploring shale gas in the North China plate. Its accumulation conditions and enrichment characteristics are quite different from that of marine shale gas fields in South China. Based on geological data such as drilling cores, logging, laboratory analysis and testing, focusing on the fracture characteristics of the Wulalike Formation, this article explores the accumulation model of marine shale gas in the northwestern margin and obtain four conclusions: (1) The Wulalike Formation, under the control of various sedimentary microfacies, develops siliceous shale, calcareous-siliceous and argillaceous-siliceous mixed shale, localized carbonate rocks (mainly brecciated limestone and marl) and calcareous shale, with large variations in lithofacies combinations in different blocks. However, the lower interval is generally rich in siliceous and exhibits similar brittleness to the lower interval of the Longmaxi Formation, which is beneficial for reservoir fracturing in the foreland thrust belt distribution zone. (2) The Wulalike Formation develops two generations of fractures: high-angle filled fractures and low-angle bedding fractures. Whilst the predominant type is the open bedding microfractures, which is more developed in the southern block compared to the northern block. (3) The lower interval of the Wulalike Formation generally has a total porosity between 2.46%-7.08%, similar to the Longmaxi Formation in the Sichuan basin, with matrix porosity accounting for 34.0%-90.0% (average 61.1%) and fracture porosity 10.0%-66.0% (average 38.9%). (4) Natural gas is mainly stored as free gas (accounting for over 64%), where primary and secondary intralayer migration occurs in inorganic pores and bedding fracture system. According to comprehensive analysis, the Majiatan-Shanghaimiao exploration area is mainly a fractured shale gas accumulation area controlled by structures. The fracture development period is synchronized with the peak gas generation period, which is conducive to the efficient accumulation of natural gas in the Wulalike Formation. The Tiekesumiao Block is a mixed shale gas reservoir.

Key words: Ordos Basin, northwestern margin, Ordovician, Wulalike Formation, fracture porosity, fractured shale gas accumulation

CLC Number:

P618.130.2

CHEN Rubiao, WANG Yuman, HUANG Zhengliang, LI Weiling, YAN Wei, LIANG Feng, GUO Wei. Fracture pore characteristics and gas accumulation model of marine shales in the northwestern Ordos Basin: A case study of the Ordovician Wulalike Formation[J]. Earth Science Frontiers, 2024, 31(5): 46-60.

Figures/Tables 17

Fig.1 Distribution of tectonic units in the Ordos Basin. Modified after [1⇓-3].

Fig.2 Composite colomn diagram for the Wulalike Formation in Well L105 in northwestern Ordos Basin. Adapted from [12]. The tri-mineral brittleness index refers to the percentage of quartz, dolomite, and pyrite in the total mineral content of rock.

Fig.3 Lithofacies division of the Wulalike Formation in the Majiatan-Dashuikeng Block. Adapted from [13]. Marine shale lithofacies types: ① siliceous shale; ② argillaceous shale; ③ calcareous shale;④ argillaceous-siliceous mixed shale; ⑤ argillaceous-calcareous mixed shale;⑥ calcareous-siliceous mixed shale; ⑦ chert; ⑧ claystone; ⑨ limestone (dolomite).

Fig.4 Photos and thin sections of rock samples from the Wulalike Formation in Well Zhongping 1, Majiatan Block

Fig.5 Relationship between total carbon (TOC) and Mo contents in the study area

Fig.6 Core production status of the study area

Fig.7 Microscopic characteristics of fractures in the Lower Wulalike Formation in Well L105

Fig.8 Microscopic observation of facture filling status of the Lower Wulalike Formation in Well 105

Fig.9 Diagram of intrusion patterns of mud invasion in the dual-track section of deep and shallow resistivity of the Wulalike Formation

Fig.10 Porosity depth profiling of the Lower Wulalike Formation in Well L105

Fig.11 Relationship between gas content and TOC in normal-pressure marine shale

Fig.12 Shale gas accumulation model for the Wulalike Formation

Table 1 Classification criteria for shale gas accumulation types. Modified after [13⇓⇓⇓⇓⇓⇓-20].

气藏类型	裂缝孔隙度/%	对应划分方案	实例
基质孔隙型页岩气聚集	<0.2	连续型页岩气聚集^[19-20]	长宁、威远、昭通、泸州、正安[龙马溪组,测试产量(2~4)×10⁴ m³/d]、彭水、Barnett等气田
裂缝型页岩气聚集	>0.5	构造型页岩气聚集^[19-20]	涪陵、Woodford、Marcellus、Haynesville、马家滩-上海庙(O₂w,忠平1井初试产量26.48×10⁴ m³/d)
混合型页岩气聚集	0.2~0.5		铁克苏庙(O₂w,棋探10井初试产量10.19×10⁴ m³/d)

Fig.13 Shale gas accumulation model for the study area. Modified after [14].

Fig.14 Profiles of shale gas reservoirs in the Majiatan-Dashuikeng area

Fig.15 Brine inclusions in high-angle filled fractures in the Lower Wulalike Formation in Well Zhong 15

Fig.16 Burial history diagram for Well Huiping 1

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