Diagenetic facies quantitative evaluation of low-permeability sandstone: A case study on Chang 8 2 reservoirs in the Zhenbei area,Ordos basin

Abstract

Quantitative characterization of diagenetic facies has great significance for reservoir evaluation and prediction. In order to find out the method to evaluate diagenetic facies, the author took Chang 8² reservoir low-permeability sandstone in Zhenbei area of Ordos basin as research object and divided the reservoir into six types of diagenetic facies by analysis of casting thin section, scanning electron microscope, cathode luminescence, and physical property. According to 14 quantitative evaluating parameters which were related with petrology characteristic, diagenesis strength, pore structure, etc. quantitative evaluation of diagenetic facies of low-permeability sandstone reservoir was done by data envelopment analysis. The result showed that in the Chang 8² reservoir low-permeability sandstone in Zhenbei area of Ordos basin, quantitative representative indexes of diagenetic facies ranged from 0 to 1.00. Various diagenetic facies and their indexes had interval corresponding relation. The diagenetic facies of weak corrosion with chlorite mat and the diagenetic facies of corrosion of unstable components had the best reservoir quality. Their diagenetic facies indexes ranged from 0.66 to 1.00. The reservoir quality of the diagenetic facies filled with kaolinite was not as good as the former. The indexes ranged from 0.50 to 0.66. The diagenetic facies of quartz secondary enlargement and the diagenetic facies of clay mineral cementation replacement had poor reservoir quality. Their diagenetic facies indexes ranged from 0.30 to 0.40. The diagenetic facies of carbonate cementation had the poorest reservoir quality. It hardly possessed fluid storage capability. After comparing diagenetic facies indexes, absorption strength and remaining oil saturability, the perfect corresponding relation between quantitative evaluation results and reservoir quality could be verified.

Keywords

Ordos basin Zhenbei area diagenetic facies data envelopment analysis reservoir quality evaluation

Introduction

Diagenetic theory is the science about diagenetic environment, diagenetic condition, diagenesis, and its formative mechanism. It studies the whole process and effect from sedimentary formation to the time before sedimentary metamorphism as well as its formative mechanisms, conditions, and final forms (Zou et al., 2008). Relative to sedimentary theory, in terms of time domain, diagenetic theory studies the series of process and effect from sedimentary formation to the time before sedimentary metamorphism. In terms of space domain, it studies the temperature and pressure conditions after sediments enter overlying reservoir, rock–fluid physical chemistry and dynamics conditions, water–rock interaction, formative mechanism and products, such as mineral compaction, new mineral crystal, protogenous mineral changes (like mineral transformation, dissolution, metasomatosis, recrystallization, and so on), deformation and fissure formed by nontectonism, and the final form through these effects and processes—diagenetic facies.

In terms of oil–gas exploration, the core work of exploring research is to find and predict advantageous reservoir. The formation of reservoir goes through two phases, one is sedimentation, and the other is diagenesis. Sedimentary theory explains the distribution of different lithologic bodies under different sedimentary environments, which is the boundary distribution of reservoir and provides basis for selection of high-quality sedimentary facies belts and distribution of reservoir. Diagenesis theory is about high-permeability space distribution of different facies and lithologic bodies, which is the inner physical property and distribution features of reservoir and provides direct basis for prediction of high-quality reservoir and quantitative evaluation of advantageous reservoir space as well as confirm the inner bound of physical property distribution of sedimentary bodies. The difference of sedimentary theory and diagenesis theory lies in that the former is external environmental conditions in favor of reservoir formation, while the latter is inner controlling factors. The two supplement each other to constitute a sound entirety for reservoir geological research and evaluating selection (Zou et al., 2008; Chen et al., 2017; Gao et al., 2015; Sun et al., 2009).

The research showed that quality of reservoir, which is also called physical property, was mainly affected and controlled by sedimentary facies, tectofacies, and diagenetic facies (Lai et al., 2013; Meng et al., 2008). In the cases that geological reservoir formation conditions like source rock and tectonic sedimentary facies were basically explicit, finding high-quality reservoir controlled by diagenetic facies was the key and difficult point of petroliferous basin later exploration and reservoir prospecting during its development (Tan et al., 2011). After deposition the sediment will enter deeper diagenetic environment under the depositional overlying reservoir and series of changes and adjustments on mineralogical composition will take place. In details, under certain temperature, pressure, and fluid, sedimentary composition and its relationship and interstitial water will have series of diagenetic changes (Zhang and Li, 2000). Sedimentary porosity evolution and physical property were mainly controlled by various kinds of diagenesis. Diagenesis is the necessary process of reservoir development and formation. It controls the evolutionary process of pore and decides the quality of reservoir (Du et al., 2012; Zou et al., 2008). However, the research reflected that it was not enough to take diagenesis impact on reservoir physical property into consideration; in fact, under the background of certain structure and deposition, formation and distribution of high-quality reservoir were also affected by diagenetic mineral, diagenetic environment, diagenetic event, and diagenetic evolution sequence. Concept of diagenetic facies was brought up for further study based on this question. Diagenetic facies highly generalized diagenesis from sedimentary formation to the time before sedimentary metamorphism and integrated influences of diagenetic mineral, diagenetic environment, diagenetic event, and diagenetic evolution sequence on reservoir physical property. Since this concept was proposed, experts have aimed at different geological features of every work area, taken good advantage of available material, and carried out fruitful research from different angles (Xu et al., 2008).

Classification and quantitative characterization methods of diagenetic facies

Classification and nomenclature of diagenetic facies

At present many experts hold different views on the definition of diagenetic facies, but most of them involve two points in common, that is diagenetic environment and diagenetic product in this environment (Zou et al., 2008). Under the background of specific depositional and chemicophysics environment, diagenetic facies is the product that went through certain diagenesis and evolution with the effect of fluid and construction (Du et al., 2012, 2006; Zhang et al., 2010). It involves comprehensive features as rock particles, cements, fabric, and fracture-cave. Based on the study of single factor diagenetic facies, experts put forward the concept of comprehensive diagenetic facies due to different academic backgrounds, research objectives, and available materials. It was defined as the integration of diagenesis with dominance in various diagenetic environments. On the basis of analyzing single factor diagenetic facies, division of diagenetic stages, cause of secondary pore, and mode of diagenetic evolution, comprehensive diagenetic facies involved effects of every single factor diagenetic facies on reservoir quality and then was divided by overlaid different single factor diagenetic facies (Du et al., 2006; Ji et al., 2008; Tan et al., 2011).

Quantitative characterization of diagenetic facies

Many experts hold different opinions on quantitative characterization of diagenetic facies. For instance, Chen and Liu (1994) took thickness of diagenetic facies and intensity of formation effect in effective reservoir space as quantitative descriptive parameters of diagenetic facies, described their geometrical morphology and size by thickness and its ratio of each kind of diagenetic facies, and used intensity of formation effect in effective reservoir space and the intensity coefficient to describe pore volume formed by various diagenesis and its relative proportion to the whole secondary pore volume. Based on the purpose of diagenetic facies evaluation, Zou et al. (2008) divided diagenetic facies evaluation into diagenetic condition analysis, observation and experimental analysis of rock core and chip, prediction of logging facies and seismic facies, comprehensive analysis, and evaluation of diagenetic facies. They also compiled relevant single factor maps and comprehensive maps. Lai et al. (2013) summarized previous research experience and thought that diagenetic synthesis coefficient was a good parameter to represent diagenetic facies quantitatively. They utilized apparent compaction percentage (α), apparent cementation percentage (β), and diagenetic synthesis coefficient (γ), which can reflect diagenesis combined effect, to evaluate diagenetic facies. They deemed that diagenetic synthesis coefficient analytical method was an ideal one to represent diagenetic facies quantitatively after combining various diagenesis impact. Li et al. (2016) studied plenty of data on physical property, chip observation, and experiments and did systematic comparative analysis. From the point of diagenesis intensity and pore structure, they selected main parameters which can represent diagenesis type and intensity, built quantitative dividing criterion of diagenetic facies in research area, and explained characteristic difference of various diagenetic facies.

The author thought that the significance of quantitative evaluation of diagenetic facies lied in providing reference for evaluating geological reservoir and data for its quantitative characterization. The key point of quantitative characterization of diagenetic facies was combining relevant parameters by an available mathematical method and forming a way in which diagenetic facies could be represented by numerical value quantitatively. Because interstitial material in research area and its content had conclusive impact on rock physical property, diagenetic facies were named and divided by main cements type, status, and diagenesis together, which controlled physical property. When it came to quantitative characterization of diagenetic facies, the author comprehensively considered with many mathematical and geological methods, such as entropy weight method, fuzzy numerical, neural network, variogram, gray and weight model, and so on. Fully considering about their advantage and disadvantage and excluding former defects, such as assigning weight subjectively, different ranges of selected parameters, and complicated dimensionalized treatment, data envelopment analysis (DEA) was finally put forward to figure out index of diagenetic facies and described it quantitatively, meanwhile, quantitative evaluation of diagenetic facies was carried out on low-permeability sandstone reservoir in Chang 8² reservoir of Zhenbei area in Ordos basin.

Geological setting of the research area

Zhenbei area is located in the junction of Shaanxi, Gansu and Ningxia provinces, southwest of Ordos basin. It started from Pengyang to Qingyang transmeridionally and from Zhenyuan to Yinjiacheng meridionally. The area stretches across Shanbei slope and Tianhuan depression (Figure 1). Structure of Chang8² is a west-dipping monoclinic which has gently gradient and partially nose-shaped structure and it belongs to deposition system of southwest delta in Ordos basin. Chang 8² reservoir is the one of main oil-bearing formations. Oil deposit is deep under the ground in 2230–2790 m.

Figure 1.

Map of the location of Chang 8² in Zhenbei area.

Table 1.

Division and research status of diagenetic facies.

Dividing basis	Author (date)	Examples
Rock mineral components	Aleta (2000)	Calcium-rich montmorillonite facies, cristobalite-rich montmorillonite facies, aragonite and high-magnesian calcite facies, low-high-magnesian calcite facies and dolomite facies, limestone facies, dolostone facies
	Liu Jian et al. (1998)
	Peters (1985)
	Ochoa (2010)
Diagenetic events	Grigsby et al. (1996)	Quartz cement facies, chlorite cement facies, calcite cement facies, mechanical compaction, cement inhibitory facies, siliceous cement, carbonate cement, clay cement
	Jennings et al. (1985)
	Elfigih (1999)
Diagenetic environment	Abercrombie et al. (1994)	High-activity monox diagenetic facies, low-activity monox diagenetic facies, gray diagenetic facies, red diagenetic facies, marine groundwater, domolite-meteoric water diagenetic facies, limestone-mudstone facies
	Lee et al. (1994)
	Tabano (1986)
	Wilson (1985)
Combined with earthquake and logging	Turner (1997)	Clay membrane cement sandstone discerned by gamma curve in northern Texas; earthquake-bottom model of diagenetic facies set for distinguishing perforated belts and compact diagenetic facies belts in Oklahoma; quantitative geology (sedimentary facies, diagenetic facies), neural network, statistical geology used for prediction of porosity in Anan oilfield
	Manthisen (1997)
	Wang et al. (1998)
Rock physics and petrographic data	Nielsen et al. (2009)	Jurassic Navajo sandstone in snow canyon national park was divided into six facies like red facies, yellow-white facies, red-white transitional facies, etc.; complete or partial diagenetic reservoir facies
Rock physics and petrographic data	Matzos et al. (1995)
Diagenesis type, strength, and diagenetic mineral	Shi et al. (2011)	Weak corrosion facies with chlorite mat, corrosion facies of unstable components
Main diagenesis, diagenetic mineral	Zhang et al. (2011)	Tight compaction-pressure facies, compaction siliceous cementation facies
Exploring practical angle and cause	Zou et al. (2008)	Low-permeability sandstone corrosion facies, compact sandstone cementation facies

Previous researches were mostly about deposition environment, reservoir features, and reservoir forming model of research area and its neighborhood. Researchers thought that reservoir had gently physiognomy and stable structure during deposition; there was no large discontinuity surface among each microfacies of deltaic plain, leading edge, and prodelta; and one set of meandering stream deltaic plain deposition formed. Deposition microfacies were underwater shunt riverway, its middle and side part, as well as riverway sandbank. Source direction stretched from west to south. The sand bodies were widespread, thick, and long. Diagenesis included constructive diagenesis (dissolution of feldspar, quartz, and cement) and destructive diagenesis (compaction, siliceous cement, authigenic clay mineral, carbonated cement, and metasomatosis).

Identification of rock slice under microscope reflected that Chang 8² reservoir was mainly constituted by granule feldspar sandstone and feldspar detritus sandstone. The proportion of granule quartz was 50% approximately, feldspar was 30%, and detritus was 20%. The main types of detritus were volcanic rock and epimetamorphic rock. Distribution of deposition detritus was little. Particle diameter was between 0.1 and 0.5 mm. Quantity of granule had absolute advantage and its volume fraction could reach 80%. The average particle granularity was 2.6. Psephicity was subangular and angular. The reservoir had good separation. The main cement type was porous cement. Cements like carbonate filled in the pores of detritus. When the cement percent was high, it would be basal cement. The supporting type was granule supporting structure. Main connection was line contact and convex–concave contact. The volume fraction of interstitial material in the research area was about 20%. The constituents of interstitial material included clay mineral like kaolinite; mica mineral like black mica, white mica, and water mica; carbonate mineral; sulfate mineral; demethicone; and a small amount of siderite and authigenic pyrite. Kaolinite, mica mineral, carbonate mineral, and demethicone were main components.

Results and discussion

The classification of diagenetic facies type

Based on the data of core observation, conventional thin section, casting thin section, and transmission electron microscopy, combining with the factors of cement content, occurrence, and diagenesis type, Chang 8² sandstone in Zhenbei district was divided into six main facies types. They were the diagenetic facies of quartz secondary enlargement, the diagenetic facies of filling with kaolinite, the diagenetic facies of carbonate cementation, the diagenetic facies of clay mineral cementation replacement, the diagenetic facies of weak corrosion with chlorite mat, and the diagenetic facies of corrosion of unstable components. Constructive diagenetic facies referred to the diagenetic facies of corrosion of unstable components and the diagenetic facies of weak corrosion with chlorite mat. Destructive diagenetic facies included the diagenetic facies of filling with kaolinite, the diagenetic facies of quartz secondary enlargement, the diagenetic facies of clay mineral cementation replacement, and the diagenetic facies of carbonate cementation (Figure 2).

Figure 2.

Reservoir characteristics for different kinds of diagenetic facies formations. (a) The diagenetic facies of weak corrosion with chlorite mat; Well Z133 core, 2359.23 m; (b) the diagenetic facies of weak corrosion with chlorite mat; Well Z159 core, 2570.70 m. (c) The diagenetic facies of corrosion of unstable components; Well Z25 core, 2450.70 m. (d) The diagenetic facies of corrosion of unstable components; Well Z279 core, 2678.00 m. (e) The diagenetic facies of filling with kaolinite; Well Z60 core, 2430.60 m. (f) The diagenetic facies of filling with kaolinite; Well Z64 core, 2580.40 m. (g) The diagenetic facies of clay mineral cementation replacement; Well Z365 core, 2492.20 m. (h) The diagenetic facies of clay mineral cementation replacement; Well Z114 core, 2700.70 m. (i) The diagenetic facies of carbonate cementation; Well Z73 core, 2492.20 m. (j) The diagenetic facies of carbonate cementation; Well Z73 core, 2496.73 m. (k) The diagenetic facies of quartz secondary enlargement; Well Z123 core, 2492.20 m. (l) The diagenetic facies of quartz secondary enlargement; Well Z123 core, 2495.30 m.

The diagenetic facies of weak corrosion with chlorite mat

In the research area, this kind of diagenetic facies matured in sedimentary microfacies environments, such as delta front underwater distributary channel, interchannel, and lateral edge. The diagenetic facies had fine granule, good separation, poor roundness, and immature sandstone. The diagenetic feature was that chlorite film developed on the edge of quartz granule and feldspar particle had corrosion entirely or partly. Primary intergranular pore was the main porous type. Authigenic chlorite enlarged rock mechanical strength, which protected pores with various origins and controlled the development of secondary quartz. The diagenetic facies of weak corrosion with chlorite mat had the best physical property in the reservoir and its porosity ranged from 12 to 16%.

The diagenetic facies of corrosion of unstable components

The diagenetic facies of corrosion of unstable components were mainly distributed in underwater distributary channel and lateral edge. The main rock property was fine and powdery sandstone which had higher matrix content. Under strong compaction the granule closely arranged and contacted linearly. The dissolved pore matured well. Secondary dissolved pore was formed due to strong corrosion of feldspar and rock debris. The porosity ranged from 10 to 16% and the physical property was good. Under the background of low permeability and porosity in the research area, this kind of diagenetic facies had constructive effect for reservoir formation and oil–gas aggregation. It was more advantageous diagenetic facies belt.

The diagenetic facies filled with kaolinite

The diagenetic facies filled with kaolinite matured in delta front underwater distributary channel. The rock property was fine grained and rock debris feldspar sandstone. Kaolinite clay mineral was common in the interstitial material of Chang 8² reservoir and its content in rock slice ranged from 1 to 4%, except for several samples which had higher content. Granule corrosion modification and porous precipitation were the main type of facies. Generally, crystalline particle was small. The precipitation of kaolinite would weaken reservoir physical property and dwindle porous quantity, which had destructive effect to reservoir property. Therefore, this kind of diagenetic facies had poor physical property.

The diagenetic facies of clay mineral cementation replacement

The clay mineral cement mainly developed in immature sedimentary facies belt of research area. It appeared as clay mineral metasomatism recrystallization (mostly illite), matrix illitization, and so on, which belonged to destructive diagenesis and could decline porosity, especially reservoir permeability. This kind of diagenetic facies had poor physical property.

The diagenetic facies of carbonate cementation

It matured on the top or bottom of thick sand body in distributary channel. The main rock property was fine and powdery sandstone which had nice separation. Calcite was the most common cement while ferrodolomite and siderite were less common than it. Carbonate crystal stock and metasomatic were the main diagenetic features. They occupied main pores completely in calcareous tight sandstone and several clastic particles got metasomatosis. Calcite content and distribution had great impact on intergranular pore and homogeneity of reservoir sand body. This kind of diagenetic facies had poor physical property.

The diagenetic facies of quartz secondary enlargement

Generally, it could be found in estuary dam and distal sand dam. The main rock property was quartz sandstone and lithic quartz sandstone. This lithofacies belt did not facilitate reservoir development. Due to the pressure solution of quartz, dissolution of silicate mineral, mutual transformation of clay mineral, and large release of SiO₂, they filled in intergranular pore in the shape of secondary concrescence of quartz edge, so that the quantity of intergranular pore decreased obviously, which was filled by autogenetic kaolinite later and developed a small amount of crystal pore. They were connected by intergranular seam. Weak corrosion would happen in some places. Sandstone porosity ranged from 4 to 8%. The pore throat was equally distributed and pore structure was good.

Quantitative evaluation of diagenetic facies

DEA was a performance evaluation method of decision-making unit (DMU) based on data, and it was also an efficiency calculation method which has been fully established and widely utilized in management science so far. DEA was developed by Charnes et al. (1978) for the purpose of solving the efficiency evaluation problems of multi-input and multioutput DMU. It was in fact through solving a special nonlinear programming to have a set of suitable multipliers of an assigned DMU, the optimal value would be used as the metric for evaluating the technical efficiency of DMU.

Charnes et al. at first only wanted to provide a method that could recognize the best unit among a set of comparable DMUs and organize an effective frontier with these units. Then, the frontier could help to calculate the efficiency levels of DMUs that were not on the frontier, and at the same time, gave a comparable benchmark for the unit. In recent years, some scholars designed many DEA models based on such nonlinear programming method to solve data analysis problems faced by many disciplines.

With accurate comprehensive quantitative evaluation of Chang 8² reservoir of Zhenbei area by using 14 diagenetic facies parameters which had influence on reservoir quality, while DEA method completely solved the problems of subjective assignments of weights and complicated dimensionalized treatment, it took advantage of relative validity of multiple inputs and outputs of its data programming model, and finally sufficed for judging and evaluating diagenetic facies correctly (Wei et al., 2004). DEA formed computing matrix of evaluation group based on diagenetic facies attribute parameters and took corresponding reservoirs of each diagenetic facies as characterization unit to determine numerical programming model which suited for diagenetic facies evaluation parameters. Model solving process and structural form adopted linear programming theory. Through analysis of single or multiple linear syntagmatic relationships of M diagenetic facies representative parameters which formed evaluation group, the final result ranged from 0 to 1.00, from which we could evaluate diagenetic facies of reservoir quantitatively. Because there was no need to give weights in advance by DEA method, subjective factors could be avoided and calculating process was simplified as well. On the basis of previous analysis about diagenetic facies parameters, 14 parameters of Chang 8² reservoir of researching area were divided into two groups: One group was that the bigger parameter values are, the better physical property of corresponding diagenetic facies reservoir is, like porosity, pore structure, permeability, and so on. The other group was that the smaller parameter values are, the better physical property is, such as self-generating calcite content and self-generated dolomite content. On this basis, taking corresponding reservoirs of each diagenetic facies as characterization unit, synthesizing DEA linear programming model, using above two kinds of parameters to get index of diagenetic facies (Q), we could characterize diagenetic facies of reservoirs quantitatively (Figure 3 and Table 2).

Suppose that randomly select n reservoir samples which were divided by diagenetic facies, each reservoir sample included s first parameters and m second parameters. During the process of DEA, the first parameters was denoted by $x_{ij} = (i = 1, 2, \dots, s; j = 1, 2, \dots, n)$ ; the second parameters was denoted by $y_{rj} = (r = 1, 2, \dots, m; j = 1, 2, \dots, n)$ .

Use DEA linear programming model to get the diagenetic facies characterization index Q. According to the associative network theory developed by Kao (2009) based on the CCR model of classic DEA’s DMU (Kao and Huang, 2008), we had

\begin{matrix} max (ω^{T} x_{o} + μ^{T} y_{j}) = Q \\ s . t . {\begin{matrix} 0 \leq ω^{T} x_{j} + μ^{T} y_{j}, 1 \leq j \leq n \\ 0 \leq ω^{T} x_{j} \leq 1 \\ - 1 \leq μ^{T} y_{j} \leq 0 \\ ω^{T} x_{o} = 1, ω \geq 0, μ \leq 0 \\ μ = tu \\ ω = tv \\ t = \frac{1}{v^{T} x_{o}} \end{matrix} \end{matrix}

Table 2.

Parameters of diagenetic facies of Chang 8² sandstones in Zhenbei area.

Sample number	nos. 1	nos. 2	nos. 3	nos. 4	nos. 5	nos. 6
Sandstone structure
Matrix (%)	10.95	12.11	11.34	11.67	10.15	11.57
Average particle size (mm)	0.39	0.21	0.27	0.23	0.17	0.22
Sorting coefficient	4.30	3.80	3.60	3.90	4.10	3.70
Skeleton particle composition
Q/(F + R)	0.87	0.92	0.88	1.07	0.97	1.06
Plasticlast	9.24	10.71	9.48	8.92	9.91	8.67
Authigenic mineral composition (%)
Chlorite	2.17	0.81	0.64	0.67	0.54	0.69
Calcite	2.78	2.16	2.36	2.27	4.01	2.11
Silica	0.97	1.01	1.21	1.29	0.97	1.23
Kaolinite	0.78	0.84	2.45	0.79	0.88	0.81
dolomite	2.11	2.03	2.07	2.01	1.47	1.97
Physical property
Porosity	15.11	11.21	8.21	6.32	7.76	3.92
Permeability	1.37	0.80	0.60	0.27	0.32	0.06
Corresponding porosity	6.70	6.80	7.00	5.30	7.20	6.68
Pore structure	1.76	1.37	0.93	0.67	0.59	0.69
Output data
Index	0.87	0.69	0.52	0.32	0.47	0.17
Diagenetic facies types	The diagenetic facies of weak corrosion with chlorite mat	The diagenetic facies of corrosion of unstable components	The diagenetic facies of filling with kaolinite	The diagenetic facies of quartz secondary enlargement	The diagenetic facies of clay mineral cementation-replacement	The diagenetic facies of carbonate cementation

Note: 1. These data were average values of each sand body datum in Chang 8² reservoir of Zhenbei area. 2. Permeability unit was 10⁻³µm², corresponding porosity referred to the porosity that 0.1 × 10⁻³µm² permeability corresponds. Its unit was %. 3. Pore structure was that surface porosity of primary pore divides by surface porosity of secondary pore.

Figure 3.

Evaluation process of data envelopment analysis of diagenetic facies.

In this formula, $x_{0} = x_{j 0}$ , $y_{0} = y_{j 0}$ , $1 \leq j \leq n$ . v was the corresponding weight coefficient of the term s type 1 input parameter $x_{j} = (x_{1 j}, x_{2 j}, \dots, x_{sj})^{T}$ of the number j reservoir sample. u was the corresponding weight coefficient of the term m type 2 input parameter $y_{j} = (y_{1 j}, y_{2 j}, y_{3 j}, \dots, x_{mj})^{T}$ of the number j reservoir sample. v and u were regional spaces that were defined by programming model and constraint condition, and the maximum product of the weight coefficient and input parameter decided the value of the weight coefficient (Huang, 2009). The maximum sum Q of $ω^{T} x_{o}$ and $μ^{T} y_{j}$ would be the diagenetic facies characterization index.

According to the principle of DEA, Q varied between 0 and 1.00. In the evaluating results, when the index of diagenetic facies was closer to 0, it indicated that the quality of reservoir in which this kind of diagenetic facies was located was worse. On the contrary, when the index of diagenetic facies was closer to 1.00, it indicated that the quality of reservoir was better.

Analysis of diagenetic facies index evaluation results

Currently, quantitative evaluation of diagenetic facies using diagenetic facies index is still at an exploration stage, and there are no set standards regarding quantitative evaluation of diagenetic facies. This paper chose 164 core samples, which covered six types of diagenetic facies, extracted the diagenetic facies parameters out of these samples, and calculated the diagenetic facies index based on the DEA method. By analyzing the calculation results, it was obvious that there was good corresponding relation between diagenetic facies types and the index, which also proved the accuracy of calculating diagenetic facies index through DEA. According to the calculation results, index ranges of various kinds of diagenetic facies for reference were listed in this paper. They were as follows: diagenetic facies of weak corrosion with chlorite mat, sample number: 1–21, index range: 0.86–1.00; diagenetic facies of corrosion of unstable components, sample number: 22–51, index range: 0.65–0.86; diagenetic facies of filling with kaolinite, sample number: 52–84, index range: 0.51–0.65; diagenetic facies of clay mineral cementation replacement, sample number: 85–105, index range: 0.40–0.51; diagenetic facies of quartz secondary enlargement, sample number: 106–129, index range: 0.30–0.40; diagenetic facies of carbonate cementation, sample number: 130–164, index range: 0–0.30 (Figure 4). According to the calculation results of diagenetic facies index, the distribution map (Figure 5) of the Chang 8² oil layer diagenetic facies in the Zhenbei area was developed in this paper by using sandstone ratio to control diagenetic facies frontier and defining types of single well reservoir diagenetic facies according to the types of dominant oil layer diagenetic facies.

Figure 4.

Diagenetic facies index distribution characteristics of number 8² of Yanchang formation in Zhenbei area.

Figure 5.

Diagenetic facies distribution of Chang 8² sandstones in Zhenbei area, Ordos basin.

Geological significance of quantitative evaluation of diagenetic facies

Diagenetic facies with a high index included diagenetic facies of weak corrosion with chlorite mat and diagenetic facies of corrosion of unstable components, and their corresponding reservoirs had the features of large porosity and wide pore throat radius, which were important factors for the formation of effective reservoirs and were in favor of productive reservoirs. However, for diagenetic facies of filling with kaolinite, the diagenetic facies index was relatively low, and the precipitation of kaolinite decreases porosity, worsened reservoir physical properties and reservoir storage capacity. Diagenetic facies of clay mineral cementation replacement had small porosity in reservoirs and the indexes ranged from 0.40 to 0.50. Cementation of different clay minerals decreased the porosity to certain extent, worsened the porosity structure, and lowered permeability; but within the target reservoir of the research area, due to the factors of diagenetic intensity and sedimentary minerals, the corresponding sand body could function for storage and migration pathway. Diagenetic facies of quartz secondary enlargement was the kind of diagenetic facies that had rather low porosity with the indexes range from 0.30 to 0.40, and this kind of diagenetic facies corresponding reservoirs had the narrowest pore and throat with low permeability. For the diagenetic facies of carbonate cementation reservoirs, the cements of calcite and dolomite in them constitute compacted calcareous deposits, which lowered the effective thickness of reservoirs, and due to the influence of ferroan calcite and dolomite, the pore and throat were blocked to a certain extent, which decreased the porosity of reservoirs, lowered the permeability and the productivity of reservoirs. The index was below 0.30, and such stratum had little fluid storage capability.

On the basis of diagenetic facies, Chang 8² reservoir of Zhenbei area was divided into four types: The first type was the diagenetic facies reservoir of weak corrosion with chlorite mat. This kind of reservoir had strong oil productivity and the highest porosity and permeability in the researching area. It was the most advantageous reservoir in the area. The second type was the diagenetic facies reservoir of corrosion of unstable components. Its oil productivity was not as strong as the first one and it also had lower porosity and permeability than the diagenetic facies reservoir of weak corrosion with chlorite mat. The third type was the diagenetic facies of filling with kaolinite. It generally belonged to destructive diagenetic facies; however, because of the existence of kaolinite a certain amount of intercrystal pores could be formed, which to some extent saved reserving spaces under the general geological ground of low porosity and permeability. Comparing with constructive diagenetic facies reservoir, it had poorer oil productivity and lower porosity and permeability. The fourth type was the diagenetic facies of clay mineral cementation replacement and the diagenetic facies of quartz secondary enlargement. Since the two kinds of diagenetic facies had similar physical properties, they were classified as the same type of reservoir in this paper. Their corresponding reservoir had the poorest oil productivity and lowest porosity and permeability in the effective reservoir of target zone in the study area.

Conclusion

Chang 8² oil reservoir group of Zhenbei area in Ordos basin was divided into constructive and destructive diagenetic facies. Constructive diagenetic facies included the diagenetic facies of weak corrosion with chlorite mat and diagenetic facies of weak corrosion of unstable components, while destructive diagenetic facies included the diagenetic facies of clay mineral cementation replacement, the diagenetic facies of quartz secondary enlargement, the diagenetic facies of filling with kaolinite, and the diagenetic facies of carbonate cementation. Using DEA method to calculate characterization indexes of diagenetic facies effectively overcame the defects of giving weights factitiously and avoided tedious process of dimensionalizing parameters when each parameter was calculated in indexes of diagenetic facies comprehensive characterization. We could calculate diagenetic facies indexes of reservoirs by quantitative evaluation of diagenetic facies. The relationships among diagenetic facies indexes, absorption strength, and remaining oil saturation could be used to evaluate the quality of reservoirs. When diagenetic facies indexes ranged from 1.00 to 0.60, they had stronger absorption strength, higher flushing efficiency, and smaller remaining oil saturation (as show in Figures 6 and 7).

Figure 6.

Relationship between the remaining oil saturation and diagenetic facies index in the Chang 8² member of Yanchang formation in Zhenbei area.

Figure 7.

Relationship between the water injection profile and diagenetic facies index in the Chang 8² member of Yanchang formation in Zhenbei area.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was funded by the Chinese National Science and Technology Major Project (No. 2017ZX05009001).

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