Distribution of quality difference of tight sandstone reservoirs in sublacustrine fan of depression lacustrine basin: A case study of Chang 6 oil-bearing interval of Yanchang Formation in Heshui area, Ordos Basin, China

doi:10.13745/j.esf.sf.2025.7.19

Abstract

Abstract:

Tight reservoirs of sublacustrine fans are currently a hot topic in continental lacustrine basin research and an important target for tight oil and gas exploration and development. This paper takes the tight sandstone reservoir of the sublacustrine fan in the Chang 6 oil-bearing interval of the Yanchang Formation in the Heshui area as the research object. By integrating core sample test data and well logging data, the distribution of matrix reservoir quality controlled by sedimentation and diagenesis was studied. The reservoir quality of the Chang 6 tight sandstone in the study area is classified into types I, II, and III. Type I reservoirs are characterized by large intergranular-intragranular pores and wide strip-shaped throats, with porosity mainly >10% and permeability mainly >0.1 mD. Type II reservoirs are dominated by small intergranular pores and wide strip-shaped throats, with porosity mainly ranging from 3% to 12% and permeability mainly from 0.01 to 0.1 mD. Type III reservoirs primarily consist of intragranular pores, micropores, and narrow strip-shaped throats, or micropores and narrow strip-shaped throats, with porosity mainly <5% and permeability mainly <0.01 mD. Type I reservoirs are mainly distributed as strips or lenses in the middle of channel branches and lobe mainbodies. Type II reservoirs, the most widely distributed type, primarily occur as sheets at the edges of channel branches and within the main parts of lobes. Type III reservoirs are mainly distributed as skirts at lobe margins. The most favorable reservoir combination, comprising the band-shaped type I and sheet-shaped type II reservoirs, is primarily developed in small layers exhibiting sheet-like sandbodies. The thicknesses of both type I and II reservoirs increase significantly with increasing small-layer sandbody thickness. When the small-layer sandbody thickness exceeds 10 m, the thickness of the type II reservoir shows a more pronounced increasing trend than that of the type I reservoir. Conversely, when the small-layer sandbody thickness is less than 10 m, type III reservoirs are significantly developed.

Key words: sublacustrine fan, tight sandstone, reservoir quality, Triassic Yanchang Formation, Ordos Basin

CLC Number:

P618.13

WANG Wurong, LIU Xianyang, YUE Dali, WAN Xiaolong, LIU Ruijing, LI Shixiang, LU Hao, LIU Jian, WU Guangzhen, WU Shenghe. Distribution of quality difference of tight sandstone reservoirs in sublacustrine fan of depression lacustrine basin: A case study of Chang 6 oil-bearing interval of Yanchang Formation in Heshui area, Ordos Basin, China[J]. Earth Science Frontiers, 2025, 32(5): 190-204.

Figures/Tables 16

Fig.1 Geological sketch map and stratigraphic characteristics of Yanchang Formation in the Heshui area of the Ordos Basin. a adapted from [26]; b modified after [32-33].

Fig.2 Distributions of composite sandbodies in sand group of Heshui area and small layer of dense well area. Adapted from [26].

Table 1 Interpretation standard of different pore-throat configurations of the Chang 6 tight sandstones in the Heshui area

孔喉组合类型	孔隙类型比例/%				喉道类型比例/%
孔喉组合类型	大粒间孔	小粒间孔	粒内孔	微孔隙群	宽片状	窄片状
大粒间孔-粒内孔- 宽片状喉道	>50	<25	>25	<25	>50	<50
小粒间孔-宽片状喉道	<25	>50	<25	<25	>50	<50
粒内孔-微孔隙群- 窄片状喉道	<25	<25	>50	>25	<50	>50
微孔隙群-窄片状喉道	<25	<25	<25	>50	<50	>50

Fig.3 Backscattered electron (BSE) images showing the characteristics of pore-throat combinations of the Chang 6 tight sandstones in the Heshui area

Table 2 Classification scheme of the reservoir quality of the Chang 6 tight sandstones in the Heshui area

储层质量分类		I类	II类	III类
储层物性	孔隙度/%	>10	3~12	<5
储层物性	渗透率/mD	>0.1	0.01~0.1	<0.01
孔隙结构特征	孔隙结构类型	I类	II类	III类
	孔隙类型	大粒间孔和粒内溶蚀孔为主,少量小粒间孔	小粒间孔为主,少量大粒间孔或粒内溶蚀孔	微孔隙群或粒内溶蚀孔为主
	喉道类型	宽片状喉道为主,偶见缩颈型喉道	宽片状喉道为主	窄片状喉道或管束状喉道为主
	孔喉组合类型	大粒间孔-粒内孔- 宽片状喉道组合	小粒间孔-宽片状喉道组合	粒内孔-微孔隙群-窄片状喉道、微孔隙群-窄片状喉道组合
	高压压汞曲线 (引自文献[33])
沉积与成岩特征	主要岩相类型	交错层理细砂岩(Sc) 中部	块状层理细砂岩(Sm) 中部	砂泥界面附近Sc和Sm岩相以及粉砂到极细粒砂岩(Ss)
沉积与成岩特征	成岩作用特征	溶解作用较强胶结作用较弱	溶解作用中等胶结作用较弱	强胶结作用或强压实作用
可动流体	可动流体饱和度/%	>35	15~40	<15
	流体可动性	较好	中等	较差
	可动流体分布模式图 (引自文献[6])

Table 3 Fisher discriminant function coefficients of the Chang 6 tight sandstone reservoir in the Heshui area

储层类别	声波时差/(μs·m^-1)	自然伽马/API	密度/(g·cm^-3)	常量
I类	12.449	-3.894	798.137	-2 275.876
II类	12.072	-3.757	801.042	-2 208.483
III类	11.891	-3.690	810.315	-2 198.242

Table 4 Fisher discrimination results of different types of the Chang 6 tight sandstone reservoir in the Heshui area

储层质量			预测结果			合计
储层质量			I类储层	II类储层	III类储层	合计
初始数据	计数	I类储层	15	2	0	17
		II类储层	10	120	19	149
		III类储层	1	5	37	43
	回判率/%	I类储层	88.2	11.8	0	100.0
		II类储层	6.7	80.5	12.8	100.0
		III类储层	2.3	11.6	86.0	100.0

Fig.4 Pie chart showing the proportion of different types of matrix reservoirs in the Chang 6 oil-bearing interval in the Heshui area

Fig.5 Interpretation results of the matrix reservoir quality of Well Z127 in the Heshui area. Modified after [36].

Fig.6 Cross-plot showing the relationship between the thickness of different types of matrix reservoirs and the sandbody thickness of small layers in the Heshui area

Fig.7 Profile dstribuion of the architeture and matrix reservoir quality perpendicular to the paleocurrent direction in the dense well area of the Heshui area

Fig. 8 Profile distribution of the architecture and matrix reservoir quality parallel to the paleocurrent direction in the dense well area of the Heshui area

Fig.9 Histogram showing the average thickness of different types of matrix reservoirs in the Chang ${{6}_{3}}^{3}$ small layer to Chang ${{6}_{2}}^{1}$ small layer in the dense well area of the Heshui area

Fig.10 Plane distribution of the matrix reservoir quality of the Chang ${{6}_{3}}^{2}$ small layer in the dense well area of the Heshui area

Fig.11 Plane distributions of the matrix reservoir quality in the dense well area of the Heshui area

Fig.12 Spatial distribution model of quality difference of tight sandstone reservoir in sublacustrine fan of depression lacustrine basin

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