Reservoir Heterogeneities between structural positions in the anticline: A case study from Kela-2 gas field in the Kuqa Depression, Tarim Basin, NW China

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 38, Issue 3, June 2011 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2011, 38(3): 282–286.

RESEARCH PAPER

Reservoir Heterogeneities between structural positions in the anticline: A case study from Kela-2 gas field in the Kuqa Depression, Tarim Basin, NW China Han Denglin1,2,*, Li Zhong3, Shou Jianfeng4, Li Weifeng1,2 1. Key Laboratory of Exploration Technologies for Oil and Gas Resources of the Ministry of Education, Yangtze University, Jingzhou 434023, China; 2. College of Geosciences, Yangtze University, Jingzhou 434023, China; 3. Institute of Geology and Geophysics, Chinese Academy of Science, Beijing 100029, China; 4. Hangzhou Institute of Petroleum Geology, CNPC, Hangzhou 310023, China.

Abstract: There are strong heterogeneous characteristics of reservoir property between the core and limbs of the Kela-2 anticline, although they are in the same structure. The reservoir heterogeneity mode of “small-scale and east-west” cannot be explained by the distribution mode of compressional stress “large-scale and south-north” proposed by previous scholars. Statistical result of petrographic composition and diagenetic characteristics shows that the differentiation of reservoir compaction between the core and limbs of the Kela-2 anticline is clear. The differentiation is controlled by the difference of tensile stress suffered by strata above the neutral plane during folding deformation. The tensile stress suffered by strata in the anticlinal core is stronger than that in the anticlinal limbs. In the Kela-2 anticline, the difference of tensile stress between the anticlinal core (Kela-201) and limbs (Kela-203 & Kela-204) is obvious. The tensile stress offsets the compaction effect caused by vertical and lateral (south-north) stress, and it is constructive to reservoir quality. It is the major factor controlling the differentiation of reservoir compaction. Key words: Tarim Basin; Kuqa depression; Kela-2 gas field; reservoir quality; structural position; compaction

Introduction An increasing number of researchers are beginning to pay attention to the structural-diagenetic alteration mechanism on siliciclastic reservoir, especially compaction effect. The research is the most active in the Kuqa depression[13]. Besides vertical compaction, there is lateral compaction (compression) on reservoirs, so that the heterogeneity of compaction effect in the reservoirs cannot be explained by the traditional theory of vertical compaction. The differences in intensity and styles of structural deformation put significant influence on reservoir quality; therefore there are strong heterogeneous characteristics of compaction effect in reservoirs with different tectonic styles. Previous researchers often analyzed the distribution of structural stress from the angle of tectonic style. The rule of reservoir pore preservation was presented in a large scale (tectonic unit). The diversity of tectonic style in the Kuqa depression is the result of different stress strengths southwards from the south margin of the Tianshan Mountain. Therefore, former research is mainly focused on discussion on “large-

scale and south-north” stress distribution, and little research is made on “small-scale and east-west” stress distribution and its restriction effect on reservoir pore preservation. With the further exploration in the Kuqa depression, more and more attentions are paid on the reservoir heterogeneity of “small-scale and east-west”. In this paper, as for the Kela-2 anticline structure in the Kuqa depression, the stress difference of “smallscale and east-west” and compaction difference are discussed.

1

Study area overview

The study area is located at the Kelasu tectonic zone in the Kuqa depression, which is in the northern margin of the Tarim Basin[4] (Fig. 1). Along with episodic uplift of southern margin of the Tianshan Mountain[57], the Kuqa depression experienced multistage tectonic activities from the Late Permian to the Neogene, which led to denudation of the Upper Cretaceous in the Kuqa depression. The Lower Cretaceous includes Yageliemu Formation, Shushanhe Formation, Baxigai Formation and Bashijiqike Formation from bottom up[5]. The Paleogene Kumugeliemu Formation overlay the Lower Cretaceous directly (Table 1).

Received date: 02 Dec. 2010; Revised date: 25 Feb. 2011. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Key Fundamental Research Project of China (2006CB202304). Copyright © 2011, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

Han Denglin et al. / Petroleum Exploration and Development, 2011, 38(3): 282–286

Fig. 1

Tectonic location of Kela-2 gas field in Kuqa depression Table 1

Cretaceous stratum of Kuqa depression

Stratigraphic Unit System Series Formation

Lithology

The lower part is thin-bedded pebbled sandstones, the middle and upper part KuPalaeoEocene mugeliemu are thick gypsum mudstone, intercagene (E1-2km) lated with siltstone and microcrystalline dolomite The part is thin/medium-thick dust-color and brown fine-median Bashijiqike grained sandstone intercalated with (K1bs) thin-bedded purple brown mudstone and muddy siltstone The middle and lower part is dust-color siltstone and fine sandBaxigai stone, the upper part are medium Creta(K1b) Lower /thick-bedded mudstone intercalated ceous with muddy siltstone Shushanhe The part is medium/stick-bedded brown mudstone and silty mudstone (K1s) The lower part is thick-bedded purple Yageliemu gray conglomerate, the upper part are sandstone, grainy sandstone interca(K1y) lated with mudstone

The Kela-2 gas reservoir is mainly the Cretaceous Bashijiqike Formation with main lithology of fine to medium grained sandstone. The Bashijiqike Formation is divided into three members from bottom up, named Ba-3 member, Ba-2 member and Ba-1 member. The Ba-2 and Ba-1 members are main gas producing layer in the Kela-2 area. Considerable efforts had been made on sedimentary environment research of the formation, and it is concluded that sandy deltas front dominates the formation. In particular, fan deltas front subfacies dominates the Ba-3 member, and the Ba-2 and Ba-1 members are braided river deltas front subfacies[8,9]. On the basis of seismic data interpretation in this area, the structure type of Kela-2 is recognized as an anticlinal fold[10,11], with brachy-axis in south-north direction. In the east-west direction, it is a broad and gentle anticline (Fig. 2a, Fig. 2b). There are two tectonic formations: Paleogene post-salt tectonic formation and Cretaceous pre-salt tectonic formation. Post-salt structures are generally Paleogene propagation folds

Fig. 2 The structure style of Kela-2 gas field and stress distribution

formed along fault detachment surface; pre-salt structures are fault anticline structures underlying detachment faults, which are gentle in south and steep in the north. The Kela-2 gas reservoir is sealed in this fault anticline.

2

Analysis and test method

In this paper, taking the Cretaceous Bashijiqike Formation (K1bs1-2) as the subject, which is the major pay zone in the Kela-2 anticline, the heterogeneous characteristics of reservoirs between different structural positions are investigated. The measured physical properties reveal that the physical properties of reservoirs in well Kela-201 in the axis of the Kela-2 anticline (Fig. 2b) are better than those of reservoirs in wells Kela-203 and Kela-204 in the both limbs of anticline. The mean porosity of the reservoir in well Kela-201 is 15.6%, that in wells Kela-203 and Kela-204 is 12.5% and 10.9%, respectively. In order to understand the difference of reservoir physical properties in the intervals, the differentiation of petrography and diagenesis for well Kela-201, Kela-203 and Kela-204 should be analysed in detail. The samples are selected from the coring section of K1bs1-2 in the above three wells. In order to ease later analysis, the samples for optical statistics are all fine sandstone with medium-good sorting and similar matrix content to minimize the influence of grain size and sorting on reservoir diagenesis. According to the methods for microscopic sandstone composition and diagenesis statistics proposed by Dickinson[12], Li Zhong[13] and Worden[14], 17 representative thin sections were selected for point-counting with no less than 300 points per thin section.

Han Denglin et al. / Petroleum Exploration and Development, 2011, 38(3): 282–286

for the compaction differentiation in the above reservoirs (Fig. 4b, 4c). 3.2

Fig. 3 Comparison between reservoir physical properties (a) and compaction effect at different structural positions in Kela-2 anticline (intergranular volume=cement content + plane porosity)

3

Compaction differentiation characteristics

The result of point-counting in thin sections indicates that the extent of compaction in the reservoir from well Kela-201 is distinctively less than that from well Kela-203 and Kela-204 (Fig. 3). The controls on the degree of compaction in reservoirs are strength of grain compressive, burial compaction and structural compaction. As following, the major controlling factor for the compaction differentiation of reservoir at different structural positions will be discussed. 3.1

Petrological characteristics

The detrital component counting shows that the sandstones in the reservoirs from different structural positions are lithic arkose mostly. It should be noted that there is little difference in between the content of both plastic grains (such as quartz and feldspar) and rigid grains (such as clay fragment) in these reservoirs (Fig. 4a,b,c). The similarity indicates that the petrographic composition is not the major controlling factor

The result of point-counting in thin sections indicates the extent of acidic dissolution in the Cretaceous sandstones are generally weaker than that in the underlying Jurassic sandstones (Yangxia and Kezilenuer Formations). The microscopic evidences from thin section study of the Cretaceous sandstones include: (1) clay coats and rims of most grains, such as quartz and feldspar grains, are in a good state of preservation; (2) there is no/less dissolution residues in the existing intergranular pores; (3) ubiquitous calcite and dolomite cements have not been dissolved, and the dissolution of analcite cement in some samples are not intense; (4) the size and distribution pattern of intergranular pores are homogeneous in the thin section scale. Therefore, good reservoirs in the research interval are developed[15], which mainly benefits from the well preservation of numerous primary pores in the reservoir. 3.3

Table 2

Pore loss caused by vertical compaction

Vertical compaction mainly refers to the compaction effect resulting from the vertical load of the overlying formation on the reservoir. For the irreversibility of vertical compaction effect, the maximum burial depth is usually used to characterize the vertical compaction effect. According to the burial history modeling in the research intervals of the three wells from different structural positions, the max burial depth are simulated, respectively. Referring to the previous experience value 0.5%/100 m[1,3] of burial compaction rate in the research intervals, the amount of pore loss caused by vertical compaction is established for the research intervals in different wells (Table 2). 3.4

Fig. 4 The characteristic of clastic composition in study reservoirs at different structural positions of Kela-2 anticline

Characteristics of pore in reservoir

Pore loss caused by non-vertical compaction

There are dual (vertical and lateral) compaction mechanism leading to pore loss in the research reservoirs. And the pore loss caused by non-burial compaction can be calculated as the whole pore loss caused by compaction deducting that caused by burial compaction (Table 2). The difference of porosity loss caused by non-burial compaction in reservoirs between structural positions is obvious, although the reservoirs are in the same structure[16,17]. In other words, the previous distribution modes of “large-scale and south-north” structural stress cannot explain the reservoir compaction differentiation in small-scale locally (well Kela-201, Kela-203 and Kela-204).

Pore loss caused by compaction in the research intervals at different structural positions in Kela-2 anticline

Well name

Intergranular volume/%

Pore loss caused by total compaction/%

Max burial depth/m

Vertical pore loss/%

Pore loss caused by non-vertical /%

Kela-201

16.8

31.2

4 774.00

23.9

7.3

Kela-203

13.9

34.0

4 947.92

24.7

9.3

Kela-204

15.4

33.3

5 052.00

25.1

8.2

Han Denglin et al. / Petroleum Exploration and Development, 2011, 38(3): 282–286

4 The relationship between structural position and reservoir compaction differentiation There is significant difference in compressional stress from different structural positions in the same fold, which is a consensus of most structure geologists but draws little attention from reservoir sedimentologists. The anticline folding and its internal strain characteristic were first proposed by Ramsay. Afterwards, many structure geologists discussed the stress distribution modes of compressional stress inside the anticline. And the stress strain of ellipsoid was considered as ideal model for attribute and strength of stress in the anticline[18] (Fig. 5). It is universally acknowledged that there is an unstretched, unshorten and undeformable stratum, named neutral plane, at about middle of deformed stratums constituting the anticline. During the deformation and folding, the stratums above the neutral were under tensile stress, and the tensional stress at the anticline axis is apparently higher than that at both limbs; the stratums below neutral plane were under compressional stress, and the compressional stress at the anticline axis is likewise higher than that at both limbs. It is obvious that the pattern and strength of stress have a direct impact on the effect of the compaction in reservoirs. It is obvious that the attribute and strength of stress have a direct impact on the effect of compaction in reservoirs. For reservoirs, the tensile stress would offset the compaction effect caused by vertical burial and lateral structure compression, which is constructive to preservation of reservoir primary pore. On the contrary, the compressional stress would increase the destruction degree of primary pore in the reservoirs. Therefore, under the same far field stress, there is an apparent difference of reservoir physical properties at different locations of anticline. As for the Kela-2 anticline, the formations constituting the anticline fold are divided into Cretaceous Yageliemu Formation, Shushanhe Formation, Baxigai Formation and Bashijiqike Formation from bottom up (Table 1). The Paleogene Kumugeliemu Formation is gypsum-salt bed with strong plasticity and was deposited above the anticline directly. The research horizon Bashijiqike Formation is the surface layer of anticline fold. On the other hand, the Shushanhe Formation is pure mudstone bed with strong plasticity also and hence acts as neutral plane during folding. Then Bashijiqike Formation, deposited above the neutral plane, was under tensile stress

Fig. 6 Sketch model of reservoir compaction differentiation at different structure positions under different tensional stress compensation in Kela2 anticline

(Fig. 6). According to the stress distribution model in the Kela-2 anticline, the constructive stress was decreasing with the distance increase to the core of the Kela-2 anticline, Therefore, in the Kela-2 gas field, comparing well Kela-201 at the structural axis to well Kela-203 and Kela-204 at both limbs, the total compaction is distinctively different due to different stress compensation (Fig. 6), which led to the reservoir heterogeneities between the core (well Kela-201) and the limbs (well Kela-203 and Kela-204 ).

5

Conclusions

There are strong compaction heterogeneity of reservoir from different structural positions in the same anticline, which cannot be explained by the distribution mode of compressional stress “large-scale and south-north” in the Kuqa depression. Furthermore, the petrographic composition and burial compaction are not the controlling factors for the above compaction differentiation. The stress differentiation from different structural positions in the same anticline is obvious. The strata above neutral plane were under tensile stress during folding deformation. Within the Kela-2 anticlinal fold, the constructive stress was the strongest in Kela-201 located in the core of anticline and decreasing towards Wells Kela-203 and Kela-204 both located in the both limbs of anticline. The differentiation of constructive stress is the major factor controlling the reservoir compaction differentiation, leading to better reservoir physical properties at the anticline axis than those at both limbs.

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