Fracture development mode in fan delta front unconventional tight oil reservoirs: A case study of Paleogene He-3 in southeastern Biyang Depression

doi:10.13745/j.esf.sf.2023.6.21

Abstract

Abstract:

The southeastern Biyang Depression is located on the steep slope of the short axis of a paleolake basin, with a complex sedimentary system of fan delta front driven by traction flow in the Paleogene. Quantitative characterization of fractures in different fabric sand bodies plays an important role in guiding unconventional tight oil exploration and development. The third member of the Paleogene Hetaoyuan Formation (He-3) in the southeastern Biyang Depression belongs to fan delta front deposit under an “uplift in depression” setting, and natural fractures have a direct influence on the adjustment of infill well pattern and the scientific formulation of enhanced oil recovery scheme. In this paper, taking the shallow middle-layer (Ⅱ-Ⅵ oil formation) continental tight sandstones of He-3 as an example, the fracture characteristics and distribution law are systematically studied using core, physical property, conventional and image logging data and numerical simulation method; the pattern of fracture development in unconventional tight oil reservoirs at the front edge of the fan delta of the study area is proposed. Lithology, sand body thickness, sedimentation and structure have significant control on fracture distribution in shallow, middle-layer tight sandstone reservoirs. Fractures are more likely to develop in single or composite thin sand bodies. Usually fractures are relatively underdeveloped when the single sand body thickness is greater than 6 m. Shallow, middle-layer fractures are mainly developed at the front of underwater distributary channel, single wing of lateral accretion channel, estuary bar and far sand bar. The river channel types are divided into homogeneous and heterogeneous. The superimposed thickness is larger in asymmetrical heterogeneous channels formed by rapid lateral accumulation, which can exceed 18 m, and the fractures are relatively developed. The homogeneous channel front, that is, the microfacies area near the fan delta front of the estuary bar, usually develops vertically in the sedimentary sequence near the top of the estuary bar, indicating strong hydrodynamic conditions and relatively developed fractures. Fractures are closely related to oil and gas accumulation. Fractures in He-3 are mainly formed in the Neogene depression period. The three-dimensional distribution of the main small-layer fractures is restored through DFN simulation. The results show that two groups of conjugate fractures are mainly developed in the E-W and NE directions, and one group is mainly developed locally. These fractures are mostly distributed parallel or perpendicular to the uplift structure. Natural fractures are more developed in the lower parts and wings of an anticline compared to its crest. In addition, the intersection of multiple conjugated fractures can form a “fracture chimeric zone,” which may be the direct cause of long-distance water channeling in the pressure flooding process, so the “fracture chimeric zone” should be avoided in the flooding process.

Key words: Biyang Depression, unconventional tight oil reservoir, deposition, structure, fracture development law

CLC Number:

YIN Shuai, ZHANG Ziyang, ZHANG Xingxing, WANG Jingchen, HU Wei, DING Wenlong, LI Hu. Fracture development mode in fan delta front unconventional tight oil reservoirs: A case study of Paleogene He-3 in southeastern Biyang Depression[J]. Earth Science Frontiers, 2024, 31(5): 139-155.

Figures/Tables 18

Fig.1 Structural characteristics and distribution of oil/gas-bearing areas in the Biyang Depression

Fig.2 Comprehensive strategraphic column diagram of the southeastern Biyang Depression

Fig.3 Porosity-permeability relationship in tight sandstone of oil layers Ⅱ-Ⅵ in He-3

Fig.4 Characteristics of fracture development in cores from Pleogene He-3 in the study area

Fig.5 Image logs revealing fracture characteristics in He-3

Fig.6 Image log interpretation from well A2132 showing fracture development characteristics in the study area

Fig.7 Stratigraphical chart and well log profiles of well A65 showing fracture development characteristics in the study area

Fig.8 Geomechanical analysis revealing positive correlation between geomechanical layer thickness and fracture spacing in the target layer

Fig.9 Cross-well profile

Fig.10 Fan-delta front depositional modes. Modified after [28].

Table 1 Comparison of sedimentation, logging response, and fracture development characteristics between low- and high-energy sand bodies in fan delta front facies

砂体类型	沉积特征	测井响应特征	裂缝发育程度
高能砂体	中-细砂岩,发育块状均质层理,处于河口坝中部及顶部或分流河道中部及底部的沉积序列,指示强水动力条件	高幅平滑箱形或钟形曲线,低伽马、较低密度值	较发育
低能砂体	中-细砂岩及粉砂岩,发育波状及砂纹非均质层理及水平层理,常见泥质条带,处于分流河道顶部或河口坝底部的沉积序列,指示较弱水动力条件	中低幅齿状、钟形或指状曲线,相对较高的伽马和密度值	发育

Fig.11 Coupling relationship between microfacies and fractures in main sublayers Ⅱ 8 1 and Ⅵ 3 2 of He-3

Fig.12 Tectonic evolution of the Biyang Depression

Fig.13 Fracture distribution of main sublayers Ⅱ 8 1 and Ⅵ 3 2 of He-3 in the study area

Fig.14 Photos showing vertical fractures of different scales in tight sandstone

Fig.15 Stress Mohr circle analysis for different rupture modes

Fig.16 Developmental patterns of fractures in tight oil reservoirs in the He-3 Member of the study area

Fig.17 Rock fracture propagation paths. Adapted from [36].

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