X凹陷A构造低阻气层成因机理分析

doi:10.11720/wtyht.2022.1416

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Abstract

As confirmed by exploration and development, many low-resistivity gas zones exist in the upper portion of layer Q₃c of the H Formation in structure A of Sag X. Given the problems such as unclear understanding of the geological sedimentary environment, insufficient microcosmic knowledge about the reservoirs, and unclear causes of the low resistivity of the gas zones, this study conducted extensive research based on the log data of three wells in the study area, as well as the data on drilling and many petrophysical experiments. Specifically, the petrological and physical property characteristics of the study area were studied using the thin-section identification data; the genetic mechanisms of low-resistivity gas zones were studied using the well tie sections and the special log data, as well as the data of many petrophysical experiments; the formation mechanisms of low-resistivity gas zones were confirmed from the microscopic visualization scale by constructing a multi-component conductivity model using the digital core technique, and the contributions of various low-resistivity geneses to the decrease in resistivity were quantitatively analyzed through the finite element-based electrical simulations. As indicated by the study results, the low-resistivity response of the gas zones in the study area is caused by the presence of clay minerals with high clay content and high cation exchange capacity and also results from the complex pore structure formed under the favorable physical property conditions in the anomalous high-pressure depositional setting. The contributions of the clay additional conductivity and the complex pore structure to the low resistivity are 35.63% and up to 64.37%, respectively. The electrical simulation results are consistent with the log-derived electrical characteristics, verifying the genetic mechanisms of the low-resistivity gas zones in the upper portion of the Q₃c.

Key words： low-resistivity gas zone clay additional conductivity pore structure digital core electrical simulation

Received: 02 August 2021 Published: 03 January 2023

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	Articles by authors
	Ren-Jie CHENG
	Jian-Meng SUN
	Jian-Xin LIU
	Peng CHI
	Xin-Di Lyu
	Wen-Liang HU
	Yan-Xin FU
	Wen-BING ZHAO

Cite this article:

Ren-Jie CHENG,Jian-Meng SUN,Jian-Xin LIU, et al. Genetic mechanisms of low-resistivity gas zones in structure A of sag X[J]. Geophysical and Geochemical Exploration, 2022, 46(6): 1369-1380.

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https://www.wutanyuhuatan.com/EN/10.11720/wtyht.2022.1416 OR https://www.wutanyuhuatan.com/EN/Y2022/V46/I6/1369

Analysis and statistics of thin sections of 3 wells in structure A

Analysis of core porosity and permeability of A-2 well

Casting thin sections of layer Q₃c(20 times eyepiece×2 times lbjective lens,single polarized light)
a—well A-2, the upper part of the Q₃c,3 600 m; b—well A-2, the upper part of the Q₃c, 3 607 m; c—well A-2, the lower part of the Q₃c,3 634 m

The inter-well profiles of 3 wells in the A structure

FMI dynamic image of Q₃c formation of Well A-2
a—FMI image of 3 596~3 602 m low-resistance gas interval;b—FMI image of 3 606~3 612 m high resistance gas interval

Comparison of X-ray diffraction-whole rock experiment results

Histogram of relative content of each clay mineral type

Comparison of microscopic features of scanning electron microscope
a—SEM of high-resistance gas layer in well A-2;b—SEM of low-resistance gas layer in well A-2;c—SEM of low-resistance gas layer in well A-2

Cation exchange capacity of various types of clay minerals

Formation pressure trend diagram

NMR logging response in Q₃c section of Well A-2

Mercury injection curve and the proportion of pores of various sizes in NMR logging
a—comparison of mercury intrusion curves of two cores with high and low resistance; b—the proportion of pores of each size in the low-barrier gas layer; c—the proportion of pores of each size in the high-barrier gas layer

Mineral composition distribution and pore network model
a—overall picture of low resistivity core; b—feldspar composition in low resistivity cores; c—illite composition in low resistivity cores; d—pore network model for low resistivity cores; e—overall view of high resistivity core; f—feldspar composition in high resistivity core; g—illite components in high resistivity cores; h—pore network model for high resistivity cores

Contents of mineral components and porosity and permeability parameters

Simulation and comparison of electrical properties of high and low resistivity cores
a—distribution of components in water-saturated high-resistance cores; b—current distribution in the case of high-resistance core saturated with water; c—component distribution of water-saturated low-resistance cores; d—current distribution in the case of water-saturated low-resistance cores; e—distribution of components of high resistivity core when water saturation is 40%; f—the current distribution of the high-resistance core under the condition of 40% water saturation; g—distribution of components in low-resistance cores when water saturation is 40%; h—the current distribution of the low-resistance core at 40% water saturation

Electrical simulation results of two digital cores with high and low resistance
a—resistivity changes of two cores with high and low resistivity under different water saturation;b—fitting diagram of resistance increase rate RI and water saturation of two cores with high and low resistivity

Variation of resistivity of 4 cores with water saturation

Analysis of the decrease in resistivity of each low resistance cause and parameter selection

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