Identification and Distribution of Carboniferous Coal Measure Source Rocks in the Bamai Area, Southwestern Tarim Basin

doi:10.19657/j.geoscience.1000-8527.2022.060

Abstract

Abstract:

In recent years, multilayer coal seams and carbonaceous mudstone have been found in the Carboniferous Karashayi Formation in the BT5 well area in the southwest of Tarim Basin. Because it is sandwiched in the mixed platform background of carbonate rock and sandstone mudstone interaction, it has not been found and reported before. Therefore, it is of great significance to find out the gas development law and distribution characteristics for finding the Carboniferous authigenic oil and gas reservoirs in this area. Based on the geochemical test of drilling cores and cuttings samples, logging lithology identification, 3D seismic inversion and so on, this paper studies the hydrocarbon generation indicators and identification distribution of the coal measure source rocks. The results show that this set of coal measure source rocks in BT5 well is related to a set of delta sedimentary system widely developed in Karashayi Formation. The maximum cumulative thickness of coal measure source rocks is about 20 m, its total organic carbon (TOC) is 10.6%-63.2%, kerogen types are of II₂-III, vitrinite reflectance (R_o) is 0.78%-1.65%, and its maturity changes with burial depth. On the whole, it is low in the north and high in the south, mainly along a set of northwest southeast delta lagoon sedimentary area. According to the AC>300 μs/m, DEN<2.3 g/cm³ and different GR values can identify the thickness and content of different types of source rocks in this area. The GR value of coal seam is less than 75 API, and the GR value of carbonaceous mudstone is 75-100 API; The GR value of shale is greater than 100 API. The distribution of source rocks can be identified in the area between wells and outside wells according to the inversion value of 3D seismic wave impedance less than 7 333 m·s^-1·g·cm^-3. The coal measure source rocks and delta reservoirs in the mixed platform are expected to form self generated and self stored oil and gas reservoirs in the Karashayi Formation in this area.

Key words: Bamai slope, Karashayi Formation, coal, delta, mixed platform

CLC Number:

ZHAO Yongqiang, SU Siyu, PU Renhai, YAO Wei, JI Tianyu. Identification and Distribution of Carboniferous Coal Measure Source Rocks in the Bamai Area, Southwestern Tarim Basin[J]. Geoscience, 2022, 36(05): 1371-1381.

Figures/Tables 11

Fig.1 Tectonic unit division map of Bamai area (a) and Upper Carboniferous to Lower Permian comprehensive histogram in Bamai area (b)

Fig.2 Comprehensive stratigraphic profile of Carboniferous Karashayi Formation of well BT5

Table 1 Determination results of TOC content, Ro and organic carbon types of different lithology in the study area

样品编号	钻井	代号	地层	来源	深度/m	岩性	T_max/℃	TOC/%	R_o/%	有机碳类型
1	BT5	C₁k	砂泥岩段	岩心	1 918.1	炭质泥岩	437	14.20	1.65	Ⅲ
2	BT5	C₁k	砂泥岩段	岩心	1 917.6	泥岩	434	0.15	0.90	Ⅲ
3	KT1	C₁k	砂泥岩段	岩屑	1 620	炭质泥岩	431	14.40	0.78	Ⅱ₂
4	BT5	C₁k	砂泥岩段	岩屑	2 040	炭质泥岩	424	10.60	0.78	Ⅱ₂-Ⅲ
5	BT10	C₁b	下泥岩段	岩屑	2 660	泥岩	436	0.01	0.80	Ⅲ
6	BT7	C₁k	砂泥岩段	岩屑	4 520	泥岩	431	0.01	0.97	Ⅲ
7	BT7	C₁k	砂泥岩段	岩屑	4 540	泥岩	428	0.86	1.33	Ⅲ
8	BT9	C₁k	砂泥岩段	岩屑	4 740	泥岩	437	1.15	1.13	Ⅲ
9	BT5	C₁k	含灰岩段	岩屑	2 020	泥岩	428	2.36	0.87	Ⅲ
10	BT5	C₁k	砂泥岩段	岩屑	2 095	泥岩	432	0.94	0.80	Ⅲ
11	BT5	C₁b	中(膏)泥岩段	岩屑	2 400	泥岩	429	0.15	—	Ⅲ
12	BT5	C₁k	砂泥岩段	岩屑	2 055	煤	425	58.10	1.29	Ⅱ₁-Ⅱ₂
13	BT5	C₁k	砂泥岩段	岩屑	2 060	炭质泥岩	424	18.50	0.81	Ⅱ₂
14	KT1	C₁k	砂泥岩段	岩屑	1 605	煤	432	63.20	0.93	Ⅲ

Fig.3 KT1-BT5-BT10 well linked cross-section in Bachu Formation (seeing Fig.1 for the well locations)

Fig.4 Contour map of coal vitrinite reflectance of the top Carboniferous Kalashayi Formation in Bamai area

Fig.5 Crossplots of AC-GR and RD-DEN logging values of different lithology of Kalashayi Formation in southwest Tarim Basin

Fig.6 Percentage diagram of coal measure source rocks in Kalashayi Formation with wave impedance less than the threshold in BT5 three-dimension seismic area

Fig.7 Isopach map of cumulative thickness of Carboniferous coal seam and Carbonaceous mudstone in Bamai area

Fig.8 Seismic section of NW-SE orientation delta progradational reflection of Karasayi Formation through BT5-BT10 well (seeing Fig.6 for section location)

Fig.9 Superposition distribution map of five-stage progradational reflections (filled with semi-translucent color)

Fig.10 Depositional facies map of Carboniferous Kalashayi Formation in Bamai area

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