Study on tight sandstone reservoir characteristics of sha 3 member in shuangtaizi structural belt of

Scientific Journal of Earth Science December 2013, Volume 3, Issue 4, PP.100-106

Study on Tight Sandstone Reservoir Characteristics of Sha-3 Member in Shuangtaizi Structural Belt of Liaohe Depression Zhufu Shao 1†, Jianhua Zhong 1, 2, Bao Liu 3, Gangshan Lin 1, Lihong Fan 1 1. School of Geosciences, China University of petroleum, Qingdao 266580, China 2. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China †Email:

kangzhu09@yeah.net

Abstract The reservoir characteristics and diagenesis are studied based on test results of conventional and casting thin sections, granularity, mercury injection, X-ray diffraction, scanning electron microscope and cathodoluminescence according to basic data. The results show that Sha-3 Member reservoir in Shuangtaizi structural belt of Liaohe Depression is very tight with a large buried depth, low porosity and permeability and poor correlation. The reservoir space types of target stratum are mainly intergranular and intercrystalline dissolution secondary pores and intergranular residual compacted primary pores, as well as intragranular dissolution pores with development of fewer fractures. The reservoir is strongly compacted with the development of argillaceous, siliceous, calcitic and ferruginous cementation. Metasomatism is mainly the replacement of quartz and feldspar by carbonate minerals. The Sha-3 Member reservoir is in the middle diagenetic stage A-B, and the middle stage A can be divided into sub-stages A1 and A2 by 3200m as the dividing line.

Keywords: Liaohe; Shuangtaizi; Sha-3 Member; Tight Sandstone; Reservoir Characteristics

1. Introduction The western Liaohe sag, located in the west of Liaohe depression, is a dustpan-shaped continental fault basin developed in Meso-Cenozoic. The sag is made up of western slope belt, transition zone of slope and sub-sag, the central uplift belt, sub-sag belt and eastern actic region distributed in turn from west to east. The north region is high and narrow while the southern part is low and wide. It covers an area of 2560 km2 at a buried depth of more than 8400m in basement. The Paleogene series develops initial rifting, strong fault depression and block-faulting depression in this basin, and the sedimentary strata shows multi-cycle. Shuangtaizi structural belt is located in the south of western sag, neighboring Qingshui sub-sag in the east and transition zone of slope and sub-sag in the west [1-5] (Fig. 1). The 3-Member of Shahejie Formation (Sha-3 Member for short) is a period of strong fault depression, with steep slope, deep water body and rich provenance. The Shuangtaizi area develops mainly the gravity-flow depositional system, including fan delta, sublacustrine fan, semi-deep lake and deep lake. The midfan subfacies in the sublacustrine fan may be further divided into braided channel, inter-channel and channel front microfacies [2, 6-9].

It develops large sets of inter-beds of grey, dark grey and beige mudstone and thick-layer lump white glutenite, at a buried depth of more than 3000 meters.

The Sha-4 and -3 Members of the Qingshui sub-sag are rich in hydrocarbon source rocks with relatively high thermal evolution of organic materials. Good source-reservoir-cap matching conditions are formed because of the direct contact between sand body and source rocks in Sha-3 Member of Shuangtaizi area. However, due to low reservoir physical property value and large buried depth, this area is a typical tight sand reservoir, especially when the buried depth is over 3500m. In addition to fast changes of sand bodies and high reservoir heterogeneity, it is necessary to have a detailed study on the tight sandstone reservoir in this area in order for a preference of favorable zones.

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FIGURE 1 LOCATION MAP OF RESEARCH AREA

2 Reservoir characteristics 2.1 Petrological characteristics The Sha-3 Member of Shuangtaizi area has multiple rock types and complex lithology [7]. Through observation to a large quantity of cores and identification to rock slices, we found that Sha-3 Member developed large sets of interbeds of thick-bed conglomerate, sandstone and dark mudstone, including boulder conglomerate, medium-coarse conglomerate, sandy conglomerate, conglomeratic sandstone, pebbly sandstone, middle-fine sandstone and mudstone. Some special structures such as convolution bedding, crumpled deformation and liquefied sandstone veins, developed in sandstone, which shows gravity flow deposition characteristics. Compositional and textural maturities of the Member are low. Its reservoir lithology is represented by feldspar lithic sandstone, minor lithic feldspar sandstone and a small amount of feldspar sandstone and lithic sandstone (Fig. 2). It is composed mainly of volcanic rock and metamorphic rock as well as small amount mudstone. Its particles, mostly in subangular-subrounded shape, present medium to poor separation, with fairly poor rounding. The particle contact types are point-line and line contacts, while the cementation types are porous and contact cements. The quantitative analyses on the whole rock mineral diffraction of four wells, namely, Shuang 51, Shuang 216, Shuangshen 3 and Shuang 225, show that there are low contents of clay and carbonate minerals which are mainly quartz and feldspar.

FIGURE 2 TRIANGULAR DIAGRAM OF SANDSTONE COMPOSITION IN SHA-3 MEMBER IN SHUANGTAIZI AREA TABLE 1 QUANTITATIVE ANALYSIS RESULTS OF WHOLE ROCK MINERAL DIFFRACTION OF FOUR WELLS FROM WORK AREA - 101 http://www.j-es.org/

Mineral

Quartz

Well

Potash

Plagioclase

Calcite

Dolomite

Clay

Feldspar Shuang 52

58.2

11.

22.7

1.5

3.1

4.9

Shuangshen 3

55.3

12.4

24.8

0.8

2.0

4.9

Shuang 216

57.6

10.5

23.0

1.3

2.4

5.2

Shuang 225

60.9

9.3

19.4

3.0

2.2

5.0

2.2 Physical properties of the reservoir The physical property analysis data of 24 wells in Shuangtaizi area were collected, including 665 porosity data and 498 permeability data. The statistical results show that the porosity ranges from 3.4 to 24.7% on an average of 13.3% (fig. 3), while the permeability changes mainly between 0.09 and 334mD on an average of 4mD. 80 percent of the porosity data points are less than 15% and 80 percent of the permeability data points are less than 8mD. As shown in Figure 4, the reservoir is mainly characterized by low porosity and permeability. Although permeability increases with porosity, it changes greatly in a poor correlation.

2.3 Space types of the reservoir The pore is a significant component of clastic rocks and an important place of oil and gas storage and migration. The space, except clastic particle, matrix, cement, and authigenic mineral in rock, can be collectively called pore space [10].

FIGURE 3 POROSITY AND PERMEABILITY DISTRIBUTIONS OF SHA-3 MEMBER IN SHUANGTAIZI AREA

In analyses of general and casting thin sections, scanning electron microscope, and cathodoluminescence, it is found that Sha-3 Member in Shuangtaizi area has various types of reservoir space, mainly including intergranular pores, intercrystal pores, and intragranular pores. Also it has a few of micro-pores, casting pores, and fractures. The intergranular pores include three types: (1) primary intergranular pores, which are the residual primary pores because of mechanical compaction and interstitial material filling; (2) intergranular dissolution pores, which are the intergranular dissolved pores after dissolution of interstitial materials (mainly carbonate cement and quartz); and (3) a small quantity of pores that are formed with edge dissolution and pressure solution of clastic particles. The intragranular pores are mainly dissolution ones unstable minerals in clastic particles, while the intragranular pores in this area are mainly the dissolved ones formed by felspar particles, and sometimes oversized casting film pores occur.

Because of the target stratum’s large buried depth and strong diagenesis, there are a small quantity of fractures which can be divided into tectonic compaction and diagenetic shrinkage fractures developing in the reservoir. Although - 102 http://www.j-es.org/

they only account for 5%, they act as reservoir space and connection channels. In this area, the reservoir space is a network system integrated with aforementioned intergranular pores, intragranular pores and fractures. Although there is a good porosity in a local area, the data analyses of permeability and mercury injection show that the reservoir bound water saturation is generally high in target stratum. Nonuniform pore structure and poor-throat connectivity in this area cause poor physical properties and tight reservoir as a whole.

3 Characteristics of diagenesis Diagenesis refers to all the changes of rock before the hard sedimentary rock changes into metamorphic rock or sustaining weathering in the process of loose deposited sediments changing into hard dimentary rocks. The research of clastic rock diagenesis is an important basis to reasonably explain the advantageous pore development zone and the formation mechanism of oil and gas reservoir space, and is also a foundation to deepen geological theory of clastic rock reservoir.

3.1 Main types of diagenesis With data analyses of conventional and casting thin sections, scanning electron microscope, and cathodoluminescence, the results show that the main types of diagenesis of Sha-3 Member in Shuangtaizi area include compaction, pressure solution, cementation, metasomatism and dissolution (corrosion). They have a significant effect on the development of reservoir pores, and lead to the periodical change of longitudinal reservoir physical properties directly [11-12].

3.1.1 Compaction After sediments undergo mechanical compaction, clastic particles will rearrange in a free stacking state to a close or tightest stacking state. These particles deform due to compression, and even rigid mineral grains are fractured or crushed. The influence factors of compaction include composition, granularity, separation and rounding, buried depth and formation pressure of particles.

The microscopic observation shows that the target stratum of study area is generally located in the strong compaction belt, where granules present a line contact (Fig. 4a).The reservoir can be divided into upper and lower sections by 3200 meters as a dividing line. The clastic particles in upper section are mainly in line contact relation (about 40%).

Besides types of transitional contact relation, spot-line and line-spot contacts are also common. More than 80% of contacts between clastic particles in the lower section are line contacts, and only few transitional contact relations can be seen. This shows that compaction intensifies gradually with depth.

3.1.2 Cementation Results of analyses on thin sections show that the sum of reservoir cement (remaining nowadays) and secondary porosity in the target stratum of study area is generally less than 20%, which illustrates the highest pore loss caused by cementation may not exceed 20%. There are many different types of cementation in different stages. Five common types of cementation are: clay mineral cementation, siliceous cementation, carbonates cementation, feldspar cementation, and pyrite cementation.

(1) Clay mineral cementation The clay mineral cementation can be specially divided into cementations of kaolinite, chlorite, and illite clay mineral.

Various kinds of clay minerals exist in particle-coating, pore-lining or pore-filling forms. The mineral content in mixed layers of illite and montmorillonite varies insignificantly with depth, and chlorite content has an unobvious increase with depth. Nevertheless, at the depth of 3100m, illite content increases while kaolinite content decreases significantly (Fig. 4b). It indicates that illite may transform into kaolinite suddenly at this depth. Meanwhile, ironand magnesium-rich diagenetic environment suitable for chlorite deposition is not common.

(2) Siliceous cementation Quartz is the most common siliceous cement in the target stratum of study area. It displays not only in authigenic enlarged edge cement of clastic quartz, but also in microcrystal granule filled in pores. And the secondary - 103 http://www.j-es.org/

enlargement quartz in the target stratum can come to Level three, revealing quite fierce diagenesis (Fig. 4c).

FIGURE 4 RESERVOIR DIAGENETIC FEATURES OF SHA-3 MEMBER IN SHUANGTAIZI AREA

(a. Well Shuang 227: 3960.82m, line contact for clastic particles; b. Well Shuang 213: 3629m, illite cementation; c.

Well Shuang 213: 2777.14m, secondary enlargement for quartz; d. Well Shuang 213: 3631.81m, dolomite crystal-stock cementation) (3) Carbonate cementation The buried depth of the target stratum is mostly over 2600m. The carbonate cements in reservoir forms in the late cementation, mainly including iron calcite and iron dolomite in crystal-stock cementation form (Fig. 4d). They often replace clasts and other components, and fill fractures and pores in the later stage. The carbonate mineral content has an increase with depth.

(4) Feldspar cementation Authigenic feldspar is also a common authigenic mineral in the clastic rocks, which exits in forms of authigenic enlarged-edge clastic feldspar or small idiomorphic crystal in matrix.

(5) Pyrite cementation.

Pyrite cement is the product in the strong reducing medium conditions, and came into being at various stages of the diagenesis. The pyrites forming in the syngenetic period or early diagenetic stage present mostly in the strawberry shape, while the pyrite forming at the diagenetic stage is grain- and nodular-shaped. The pyrite in the target stratum of study area is generally strawberry-shaped, and it is the cement in the early diagenesis.

3.1.3 Metasomatism Metasomatism refers to a phenomenon that primary minerals in clastic rocks are replaced by epigenetic ones. In essence, the dissolution of replaced minerals and the deposition of replacing minerals occur simultaneously, and the placed minerals are replaced gradually. The most remarkable behavior in the target stratum of study area is the metasomatism of different rock structure components by (iron-containing) carbonate minerals in the late stage. It may occur in clay cement of the edge and the inside of particle or inter-particle.

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3.1.4 Dissolution Dissolution refers to the dissolution of underground water to rock components, and it starts with particle surface or the crack of particle and interstitial material till to the particle and interstitial material inside gradually. Dissolution is an important way to improve porosity and permeability conditions of sandstone reservoir.

Corrosion to feldspar particles is the most common dissolution in the target stratum of study area, and the dissolution of other structural components is sporadic. With the buried depth, more and more feldspar components are dissolved, causing precipitation of derivative minerals. The quantitative analysis of X-ray diffraction reveals that total quantity of feldspar decreases with depth gradually while the clay minerals increase with the depth. This shows the dissolution process of feldspar.

3.2 Division of diagenetic stage 3.2.1 Sequence of diagenetic evolution According to the diagenesis types and depth relationship, the sequence of diagenesis can be roughly determined as follows: early carbonate cementation, authigenic clay mineral cementation and strawberry-shaped pyrite cementation —early carbonate and feldspar dissolution—quartz’s secondary enlargement, cementation of microcrystalline quartz and authigenic clay mineral (kaolinite), and feldspar’s authigenic enlargement—late carbonate cementation— dissolution of late carbonate, feldspar and other cements. It should be noted that the compaction has been continuing during the entire process of diagenetic evolution sequence. Early carbonate cementation not only inhibits compaction, but also provides material source for later dissolution.

3.2.2 Division of diagenetic stage According to comprehensive various analyses and test data in target area of the study area, as well as previous relevant studies, the diagenetic stage of target stratum is divided in this paper. The items for comprehensive studies include pore types, particle contact relationship, dissolution features, sandstone authigenic mineral characteristics, argillaceous rock compositions, vitrinite reflectance, organic material pyrolysis peak temperature, paleo-temperature, buried depth, etc.. Sha-3 Member of study area is located generally below 2500m. Its diagenetic stage is equivalent to stage A-B of the middle diagenetic phase according to its current diagenetic features. Stage A can be further subdivided into sub-stages A1 and A2 by 3200m as the dividing line roughly. The diagenetic features of the two sub-stages are quite different (Fig. 5).

FIGURE 5 DIVISION CHART FOR DIAGENETIC STAGE OF SHA-3 MEMBER IN SHUANGTAIZI STRUCTURAL BELT - 105 http://www.j-es.org/

4 Conclusions (1) The reservoir lithology is very complicated for Sha-3 Member in the Shuangtaizi structural belt of Liaohe Depression. Its particle grade varies from boulder conglomerate to siltstone. The member has lower compositional maturity and structural maturity, poor reservoir physical property, poor porosity and permeability correlation, but strong heterogeneity.

(2) The Sha-3 Member reservoir develops mainly secondary dissolution pores, as well as intergranular primary pores.

Dissolution pores include intergranular (intragranular) pores, grain dissolution pores, and cement dissolution pores.

(3) The main reservoir diagenesis includes compaction, cementation, metasomatism and dissolution. Calcium, silica, iron, and argillaceous cements coexist. The dissolution occurs mainly in the feldspar and carbonate cements, and develops mainly the replacement of carbonate minerals towards quartz and feldspar.

(4) The target stratum presents a diagenetic sequence, namely, early cementation, early dissolution, authigenic enlargement of quartz and feldspar, late cementation and late dissolution. The diagenetic stage is equivalent to stage A-B of the middle diagenetic phase, where, Stage A can be subdivided again into sub-stages A1 and A2 at 3200 meters as the dividing line.

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