Genetic Mechanism of Low-Resistivity Neogene Zhujiang Formation in Wenchang X-2 Oilfield of Pearl River Estuary Basin

doi:10.19657/j.geoscience.1000-8527.2020.02.17

Abstract

Abstract:

A set of low-resistivity oil reservoirs is developed in the Zhujiang Formation of Wenchang X-2 Oilfield in the Pearl River Estuary Basin. Resistivity of the oil reservoirs is very close to that of the water layers, which creates challenges in identifying reservoir fluids and calculating water saturation. At present, the genetic mechanism of these low resistivity oil reservoirs is yet to be clear. Based on mercury injection, nuclear magnetic resonance, casting thin section, CT scan, X-ray diffraction and other experimental analyses, and integrated with geological and well-log data and electrical conductivity numerical simulation technology, the genetic mechanism is analyzed from both macro- and micro-scale perspectives. The results show that if the banded shale content is above 20%, the rocks (with the shale) have a good continuity and can provide good conductive network. The resistivity of the oil reservoirs is below 1.6 Ω·m, and the resistivity differences between oil reservoirs and water layers are not distinct. Low-amplitude structures and shallow-sea sedimentary environment are the major macroscopic causes of the low-resistivity oil reservoirs. Complex pore structure, fine rock granularity, high irreducible water saturation, high banded-shale content are the major microscopic causes of low-resistivity oil reservoirs.

Key words: Pearl River Estuary Basin, Zhujiang Formation, low-resistivity oil reservoir, shaly sandstone, sedimentary environment, pore structure

CLC Number:

TE311

HU Xiangyang, LIANG Yunan, WU Feng, LIAO Mingguang, ZHANG Hengrong, YANG Dong, YANG Yi, DAI Jin, ZHONG Huaming, WU Yixiong. Genetic Mechanism of Low-Resistivity Neogene Zhujiang Formation in Wenchang X-2 Oilfield of Pearl River Estuary Basin[J]. Geoscience, 2020, 34(02): 390-398.

Figures/Tables 9

Fig.1 Structures and stratigraphic characteristics of Pearl River Estuary Basin

Fig.2 Resistivity curves of low-/high-resistivity oil reservoirs

Fig.3 Relationship between sedimentary environment and reservoir resistivity in Zhujiang Formation

Fig.4 Sedimentary microfacies in low-/high-resistivity oil reservoirs

Table 1 Rock grain-size statistics in low-/high-resistivity oil reservoirs

岩石颗粒	粒径/mm	低阻油层岩石颗粒相对含量/%	高阻油层岩石颗粒相对含量/%
粗砂	2.000 0~0.500 0	<0.005	0.001~14.43
中砂	0.500 0~0.250 0	0.005~3.47	0.001~26.71
细砂	0.250 0~0.062 5	7.25~25.60	12.00~57.90
粉砂	0.062 5~0.003 9	53.93~72.05	24.59~53.82
黏土	<0.003 9	11.92~25.23	9.39~16.30

Fig.5 Types and contents of clay minerals and their SEM imaging characteristics in low-/high-resistivity oil reservoirs

Fig.6 Micro CT images and cast thin section images of low-resistivity oil reservoirs

Fig.7 Thin section image-based conductivity models and electrical current density distributions in low-resistivity oil reservoirs

Fig.8 Comparison of the simulation results in low-/high-resistivity oil reservoirs and well-log data

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