Controls of the Degree of Order of Dolomite on the Physical Properties of Lacustrine-facies Diamictitic Carbonate Reservoirs:A Case Study of the Yingxi Area in the Qaidam Basin

doi:10.19657/j.geoscience.1000-8527.2019.03.16

Abstract

Abstract:

To study the physical properties and main controlling factors of the Oligocene lacustrine-facies diamictitic carbonate reservoirs in Yingxi area, we have conducted systemic analyses on the petrology and mineralogy, physical properties, pore structure, degree of order of dolomite, formation mechanism of the disordered dolomite and its influence on the reservoir physical properties. It is suggested that the Oligocene reservoirs in the Yingxi area is mainly of low porosity and ultra-low permeability, and its pore development is mainly controlled by the dolomite content in the rocks. The degree of order of dolomite likely plays an important role in controlling the physical properties of the rocks. Our results also show that: (1) the well-log porosity is positively correlated with the dolomite content of the rocks, but negatively correlated with the clay content; (2) when the dolomite content is 100%, the molecular structure of the ordered dolomite suggests that its porosity should be 13% in the ideal model. In the correlation diagram of the measured porosity (via the helium gas method) versus the dolomite content, most of the data points are distributed near the ideal pattern line; (3) the intercrystal pores above the ideal pattern line are expanded, some are enlarged by the support of clastic particles, and some by weak dissolution; whereas the intercrystal pores below the ideal pattern line are mostly filled with pyrite or other material; (4) a few rock samples have relatively high dolomite content but smaller porosity. They are mainly composed of dolomite with low-/zero-degree of order. With the same dolomite content, the higher the degree of order is, the better developed are the pores.

Key words: degree of order, dolomite, intercrystal pores, physical properties, diamictitic carbonate rock, Yingxi area, Qaidam Basin

CLC Number:

TE122

DU Jiangmin, WANG Qingchun, NIE Wancai, LUO Chuanyou, LI Zheng, SHENG Jun. Controls of the Degree of Order of Dolomite on the Physical Properties of Lacustrine-facies Diamictitic Carbonate Reservoirs:A Case Study of the Yingxi Area in the Qaidam Basin[J]. Geoscience, 2019, 33(03): 643-652.

Figures/Tables 10

Fig.1 Oligocene sedimentary pattern of the Yingxi area in the Qaidam Basin

Table 1 Industry standard classification on reservoir porosity and permeability type of carbonates

孔隙度类型	孔隙度?/%	渗透率类型	渗透率K/10^-3 μm²
高	?≥20	高	K≥100
中	12≤?<20	中	10≤K<100
低	4≤?<12	低	1≤K<10
特低	?<4	特低	K<1

Fig.2 Correlation diagram of well-log porosity vs.dolomite and argillaceous contents in the Oligocene rock from Yingxi well S41-2

Fig.3 Dolomite crystal morphology and formation model of intercrystal pores of the Oligocene reservoirs at Yingxi area

Fig.4 Correlation diagram between the measured porosity (via helium gas method) and the dolomite content of the Oligocene rocks in Yingxi area

Fig.5 Expansion and filling characteristics of intercrystal pores in the Oligocene dolomite reservoirs in Yingxi area

Fig.6 Schematic sketch of dolomite lattice

Table 2 Degree of order of the Oligocene dolomite in the Yingxi area

井号	深度/m	层位	氦孔隙度/ %	视密度/ (g/cm³)	白云石含量/ %	I(015) 反射强度	I(110) 反射强度	白云石有序度
S41-2	4 197.24	$E 3 2$	1.705	2.693 189	65.5	0	670	0.00
S41-2	4 198.11	$E 3 2$	1.276	2.749 779	70.0	0	721	0.00
S41-2	4 198.80	$E 3 2$	1.848	2.684 673	63.1	0	575	0.00
S41-2	4 198.92	$E 3 2$	1.785	2.698 383	64.6	228	683	0.33
S41-2	4 203.61	$E 3 2$	1.72	2.686 639	68.2	304	654	0.46
S41-2	4 204.05	$E 3 2$	1.858	2.678 940	60.8	310	695	0.45
S41-2	4 205.69	$E 3 2$	1.556	2.688 074	71.3	0	737	0.00
S41-2	4 205.79	$E 3 2$	1.604	2.701 509	63.6	0	674	0.00
S41-2	4 206.48	$E 3 2$	1.588	2.728 038	75.7	321	880	0.36
S41-2	4 206.92	$E 3 2$	1.675	2.720 311	77.3	230	691	0.33

Table 2 Degree of order of the Oligocene dolomite in the Yingxi area

井号	深度/m	层位	氦孔隙度/ %	视密度/ (g/cm³)	白云石含量/ %	I(015) 反射强度	I(110) 反射强度	白云石有序度
S41-2	4 197.24	$E 3 2$	1.705	2.693 189	65.5	0	670	0.00
S41-2	4 198.11	$E 3 2$	1.276	2.749 779	70.0	0	721	0.00
S41-2	4 198.80	$E 3 2$	1.848	2.684 673	63.1	0	575	0.00
S41-2	4 198.92	$E 3 2$	1.785	2.698 383	64.6	228	683	0.33
S41-2	4 203.61	$E 3 2$	1.72	2.686 639	68.2	304	654	0.46
S41-2	4 204.05	$E 3 2$	1.858	2.678 940	60.8	310	695	0.45
S41-2	4 205.69	$E 3 2$	1.556	2.688 074	71.3	0	737	0.00
S41-2	4 205.79	$E 3 2$	1.604	2.701 509	63.6	0	674	0.00
S41-2	4 206.48	$E 3 2$	1.588	2.728 038	75.7	321	880	0.36
S41-2	4 206.92	$E 3 2$	1.675	2.720 311	77.3	230	691	0.33

Fig.7 XRD spectra of the Oligocene disorder dolomite in the Yingxi area

Fig.8 SEM images of the Oligocene disorder dolomite in the Yingxi area

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