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Characteristics, origin and distribution of dolomite reservoirs in Lower-Middle Cambrian, Tarim Basin, NW China
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 43, Issue 3, June 2016 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2016, 43(3): 375–385.
RESEARCH PAPER
Characteristics, origin and distribution of dolomite reservoirs in Lower-Middle Cambrian, Tarim Basin, NW China SHEN Anjiang1, 2, ZHENG Jianfeng1, 2,*, CHEN Yongquan3, NI Xinfeng1, 2, HUANG Lili1 1. PetroChina Hangzhou Research Institute of Geology, Hangzhou 310023, China; 2. Key Laboratory of Carbonate Reservoir, CNPC, Hangzhou 310023, China; 3. PetroChina Tarim Oilfield Company, Korla 841000, China
Abstract: Based on cores, thin sections and drilling data of 18 wells and two outcrop profiles of the Lower-Middle Cambrian in the Tarim Basin, geochemical analysis of multi-parameters in micro-area (the micro-carbonate fabric which is formed under the same diagenetic event or has the same genesis) and reservoir dissolution modeling were carried out to find out the types, origin and distribution of Lower-Middle Cambrian dolomite reservoirs. There develop three types of dolomite reservoirs, margin reef-shoal reservoir, platform interior mound-shoal reservoir and platform interior gypsodolomite reservoir. The rock types include algae dolomite, grain dolomite, and gypsodolomite; the pore types in them include algae framework pores in algae dolomite; intergranular pores, intra-granular dissolved pores, inter-crystalline dissolved pores in grain dolomite and gypsum-dissolved pore gypsodolomite. The primary pores in sediments of reef-shoal facies and gypsodolomite flat are the key to the development of the dolomite reservoirs, some aragonite and calcite are the material basis of dissolution in early supergene stage, and the dolomite formed in penecontemporaneous dolomitization acts as strong rock skeleton to protect the primary pores from destructive effect in burial stage. The margin reef-shoal reservoirs, large in scale and good in physical properties, are the most practical exploration targets; furthermore, the platform interior reef-shoal reservoirs in Lower Cambrian Xiaoerbulake Formation and platform interior gypsodolomite reservoir in the Middle-Lower Cambrian have higher oil and gas potential. Key words: Tarim Basin; Lower-Middle Cambrian; dolomite reservoir; margin reef-shoal; platform interior reef-shoal; platform interior gypsodolomite
Introduction With the deepening of understanding on the Lower Cambrian Yuertusi Formation source in Tarim Basin[1], Well Zhongshen1 and Zhongshen5 have been drilled subsequently since 2012, in which, the Lower Cambrian interval between 6 439–6 458 m in Well Zhongshen1 was tested an open flow of 5.10–15.40 m3 oil and 1 731–10 301 m3 gas a day with 3 mm choke; and the lower Cambrian 6 597.63–6 785.00 m was tested an open flow of 30 281 m3 gas and 34.92 m3 water a day with 5 mm choke, which unveiled the prelude of oil and gas discovery below the Cambrian salt. The lower Cambrian 6 861–6 944 m in the sidetracked Well Zhongshen1C was tested an open flow of 158 545 m3 gas a day with 5 mm choke, marking a strategic breakthrough in the exploration of dolomitic reservoir below the Cambrian salt[2]. Although exploration breakthrough has been made in the exploration of Lower Cambrian dolomitic reservoir in the basin, poor understanding in its reservoir type, origin and
distribution still hinders play and target assessment. Based on cores, thin sections, and single well data from 18 drilled wells (Yaha 5, Yaha 10, Yaha 7X-1, Xinghuo 1, Yingmai 7, Yingtan 1, Tong 1, Fang 1, 4, and 6, Shutan 1, Batan 5, Kang 2, Mabei 1, Zhongshen 1, Zhongshen 5, Tacan 1) encountering the Lower Cambrian, geochemical analysis of multi-parameters in micro-area (the same type of diagenetic minerals formed in different diagenetic events) and reservoir dissolution modeling were carried out to find out the types, characteristics and origins of the Lower-Middle Cambrian dolomitic reservoirs intra-salt and below-salt, and the distribution of scale dolomitic reservoir, to provide a basis for play assessment and target prospecting.
1. 1.1.
Geology setting of reservoir development Stratigraphic sequence and features
Middle-Lower Cambrian in Tarim Basin can be divided into platform, slope and basinal sedimentary facies systems[3], in
Received date: 05 Aug. 2015; Revised date: 25 Mar. 2016. * Corresponding author. E-mail: [email protected] Foundation item: Supported by China National Science and Technology Major Project (2016ZX05004-002); PetroChina Science and Technology Major Project (2014E-32-02). Copyright © 2016, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
SHEN Anjiang et al. / Petroleum Exploration and Development, 2016, 43(3): 375–385
in which reservoirs mainly develop in the platform area, upwardly, the strata sequence and lithofacies in the Cambrian (Fig. 1) are as follows: the Yuertusi Formation distributing in gentle slope and in basinal zone, is mainly composed of gray-green, gray-black mudstone, intercalated with thin-bedded limestone and diatomite, with a total organic carbon value (TOC) of 1.00 %–9.43%, averaging 5.5%, and a thickness of about 28 m; the lower part of the Xiaoerbulake Formation is mainly composed of gray-black dolomicrite and powdered crystal dolomite, while the middle-upper part of the Xiaoerbulake Formation mainly contain brown gray-gray grid algae, algae psammitic dolomite, and a small portion of dolomicrite, with a thickness of about 204 m, in which the reservoir is mainly in platform margin and inter-platform reef and shoal sedimentary facies; the Wusongge’er Formation about 103 m thick is characterized by large sets of dark gray-dark gray argillaceous dolomite, gypsodolomite, and dolomitic gypsum interbedding with a small clip of psammitic dolomite formed in arid climate, in which the reservoirs are gypsodolomite and psammitic dolomite; in the about 237 m thick Shayilike Formation formed in the evaporation climatic background, the upper part of the Shayilike Formation mainly contains gray-brown, dark-gray limestone, the middle-lower part mainly gray to brown gypsum, intercalating with purple mudstone, argillaceous dolomite, gypsum dolomite and a minor of psammitic dolomite, the reservoir is mainly in gypsodolomite and psammitic dolomite; in the Avatage Formation about 326 m thick formed in evaporation climatic background, the upper Avatage Formation mainly contains gray brown, dark gray argillaceous dolomite, marl, intercalating with dolomitic gypsum and a minor of psammitic dolomite, the middle-lower part is mainly gray, light gray brown gypsodolomite and a minor of psammitic dolomite, and the reservoir is mainly in gypsodolomite and psammitic dolomite; the Qiulitage Formation about 722 m thick mainly contains gray, dark-gray psammitic dolomite, dolomicrite, intercalating with marl dolomite. 1.2.
Sedimentary facies and paleogeography
Based on quantitative well logging of texture components of carbonate and seismic identification technology for carbonate lithofacies, the lithofacies paleogeography of Tarim Basin in Early-middle Cambrian has been delineated. In Early Cambrian, the Tarim Basin was a gentle slope-weakly rimed platform on the whole, the development of paleocontinent located in southwest Tarim Basin controlled the type and trending of sedimentary facies. Formed in semi-arid climate setting, the platform was mainly comprised of turbidite tidal flat, gypsodolomite flat, inter-platform shoal, and in-platform depression, with the sedimentary belts mainly trending NW-SE; and there mainly developed slow slope-weakly rimed platform margin (Fig. 2), algal reef (mound), and granulocyte debris shoal in the north and east part of the platform. In Middle Cambrian, the basin was a rimed platform with much more arid climate than that of Early Cambrian, and was char-
Fig. 1.
Stratigraphic column of the Cambrian in Tarim Basin.
acterized by the development of four different gypsum lacustrine sedimentary systems from the centre outward sequentially, the gypsum dolomitic flat-(argillaceous) dolomitic flat-platform margin depositional system belt (Fig. 3), gypsum saline lacustrine and gypsum dolomitic flat formed in arid climate comprised the main body of the inter-platform depositional system.
2. Types and characteristics of dolomite reservoir There develop three types of reservoirs in the Middle-Lower Cambrian in Tarim Basin, platform margin reef flat, inter-platform mound-shoal, and inter-platform gypsodolomite, in which the inter-platform mound-shoal reservoir can be subdivided into two types, buried hill and inside-hill, the reservoir is obviously controlled by the distribution of sedimentary facies. 2.1.
Platform margin reef shoal reservoir
Limited by the cores from drilling well, the study takes platform margin reef and shoal reservoir in the Xiaoerbulake Formation of Sugetbulak section in Keping region as an example. According to the characteristics of lithology and tested natural gamma (Fig. 4), the Xiaoerbulake Formation of
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Fig. 2.
Fig. 3.
Lithofacies and paleogeographic map of Tarim Basin in Early Cambrian.
Lithofacies and paleogeographic map of Tarim Basin in Middle Cambrian.
Sugetbulak section can be divided into two members, the upper and lower members, of which the upper member can be subdivided into the first, second, and third submembers. The Lower Member of Xiaoerbulake Formation mainly made up of thin-bedded gray black dolomicrite and fine crystal dolomite and pellet-fine crystal dolomite, is fairly stable in thickness and dense in lithology. The first submember of Upper Member fairly stable in thickness, mainly consists of gray-dark gray thin-bedded psammitic dolomite and fine crystal dolomite, gray algal-layered dolomite (intercalated with algae psam-
mitic dolomite lenticles) and algae psammitic dolomite, with small amount of pores in the algae psammitic dolomite. The second submember of the Upper Member of Xiaoerbulake Formation has thick psammitic dolomite, and pore and cavity increasing upward in the lower part, thick algal skeleton dolomite and widespread dissolved pores and cavities in the middle, a set of thick psammitic dolomite and stromatolitic dolomite, acicular dissolved pores increasing upward in the top. The third submember of the Upper Member of Xiaoerbulake Formation mainly consists of thin- to medium-layered
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Fig. 4.
Composite sedimentary column of the Xiaoerbulake Formation in Sugetbulak Section.
psammitic dolomite-bearing dolomicrite and fine crystal dolomite, medium-layered fine crystal dolomite, with pores in the top. Song Jinmin et al. described in detail the biological structure of the reef and shoal in the Upper Member of Xiaoerbulake Formation, and considered the reef and shoal was micro-organism genesis[4], this study also shows that the development of shoal and reef reservoir in the Lower Cambrian Xiaoerbulake Formation is related to fungi and algae. The second submember of the Upper Member of Xiaoerbulake Formation, about 47.5 m thick, is the main interval of reef and shoal, and also the major interval of reservoir (Fig. 4).
It is characterized by big shoal and point reef in the lower part, large reef in the middle and large shoal in the upper part. Pores are most developed in the algal sand debris shoal, algae skeleton reef in the upper part and algal stromatolite at the top. The pore types include algae skeleton, dissolved cavity, and intergranular pores. The algae skeleton pore mainly distributing in algal reef dolomite, is a typical kind of primary pore, partially filled by dolomite cement (see Fig. 5a). Dissolved pores, intergranular (dissolved) pores are mainly found in powdered crystal-fine crystal dolomite (particles phantom structure), algae psammitic dolomite, with selective fabric
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dissolution feature (Fig. 5b). Algae psammitic shoal dolomite has a porosity of 1.90%–9.39%, 5.50% on average, low energy beach between the reef has a porosity of 0.85%–4.10%, 1.76% on average, algal reef has a porosity between 1.32% and 10.55%, 4.81% on average. Quantitative CT analysis of 25 mm algal dolomite samples shows that the samples have various sizes of algal skeleton pores, unobvious differentiation between pores and throats, and 56.20% connected volume and good pore-throat connectivity (Fig. 6a, 6b). 2.2.
Platform internal mound-shoal reservoir
The Lower Cambrian inter-platform algal mound dolomitic reservoir in Well Fang 1 and the Middle Cambrian platform psammitic shoal dolomitic reservoir in Well Yaha 7X-1 are taken as examples to illustrate the characteristics of inter-platform mound-shoal reservoir. The 4 598.2–4 606.8 m in Well Fang1 of the Lower Cam-
brian Xiaoerbulake Formation is algal mound dolomitic reservoir, in which pores include mainly algal skeleton residue pores (Fig. 5c), and a small amount of gypsum-dissolved vugs. According to 27 samples analyzed, it has a porosity between 0.76%–3.57%, 1.55% on average, in which, samples with the porosity between 2.50%–3.57% account for 15.38%, samples with the porosity of 1.50%–2.5%, 23.38%, samples with porosity of less than 1.5%, 61.24%. Compared with platform margin algal reef reservoir in the Xiaoerbulake Formation in Sugaitebulaike section, generally, the inter-platform algal mound has poorer physical properties. The 5 815–5 840 m interval in the Cambrian Avatage Formation of Well Yaha 7X-1 is psammitic dolomite reservoir, in which pores include intergranular pores and intragranular dissolved pores, a small amount of dissolved pores (Fig. 5d). The reservoir has a porosity between 0.85%–6.97%, 4.6% on average, samples with porosity between 2.5%–4.5% account
Fig. 5. Types and characteristics of dolomite reservoir in the Lower Cambrian, Tarim Basin. (a) Algal dolomite, rich in framework pores, and the peripheral filled by early dolomitic cement, blue cast thin section, Xiaoerbulake Formation, in the Sugetbulak section; polarized light; (b) Psammitic dolomite, rich in dissolved pores and vugs along the stratum, outcrop samples of Xiaoerbulake Formation, in the Sugetbulak section; (c) Algal reef (mound) dolomite, rich in algae framework pores, the peripheral filled by minor early dolomitic cement and gypsum, Well Fang 1, Xiaoerbulake Formation; 4 599.00 m, red cast thin section, polarized light; (d) Psammitic dolomite, rich in inter- and inter-granular dissolved pores, Well Yaha 7X-1, Avatage Formation, 5 832.00 m, blue cast thin section, polarized light; (e) Psammitic dolomite, intergranular dissolved pore, Well Kang 2, Xiaoerbulake Formation, 5 497.00 m, blue cast thin section, polarized light; (f) gypsum-bearing dolomicrite, casted molds formed by gypsum concretion dissolution, Well Yaha 10, Avatage Formation, 6 170.95 m, blue cast section, polarized light; (g) Gypsum-dissolved breccia, rich in inter-breccia pore, Well Yaha 10, Avatage Formation, 6 210.40 m, red cast thin section, polarized light; (h) Micro-pore in dolomicrite, Well Zhongshen 5, A Watage Formation, 6 197.70 m, blue cast thin section, polarized light; (i) Gypsum-bearing dolomicrite, rich in micro-pore, Well Zhongshen 5, Wusongge’er Formation, 6 600.00 m, scanning electron microscopy.
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Fig. 6. Pore and throat structure of the dolomitic reservoir in Lower Cambrian, Tarim Basin. (a) CT three-dimensional imaging photo of a sample taken from Sugetbulak outcrop, Xiaoerbulake Formation; (b) the sample is same as (a), CT-based analysis of pore and throat radius distribution; (c) CT three-dimensional imaging photo, Well Yaha 7X-1, Avatage Formation, 5 832.00 m; (d) the sample is same as (c), CT-based analysis of pore and throat radius distribution; (e) CT three-dimensional imaging photo, Well Yaha 10, the Avatage Formation, 6 170.95 m; (f) the sample is same as (e), CT-based analysis of pore throat radius distribution; (g) CT three-dimensional imaging photo, Well Zhongshen 5, Wusongge'er Formation, 6 600.00 m; (h) the sample is same as (g), CT-based analysis of pore and throat radius distribution.
for 21.43%, and samples with porosity below 4.5% account for 64.29%. CT scanning analysis of 25 mm psammitic dolomite samples shows relatively uniform distribution of pores and throats, with vertical cyclicity, quantitative analysis shows little differentiation between pores and throats, and fairly good connectivity (Fig. 6c, 6d). Compared with Well Fang 1, it is found that the inter-platform psammitic shoal reservoir has much better physical property than algal mound reservoir. In addition, this kind of reservoir is also frequently seen in the Xiaoerbulake Formation, for exmaple Well Kang 2 (Fig. 5e) etc. 2.3.
Inter-platform gypsodolomite reservoir
Inter-platform gypsodolomite reservoir can be subdivided into two types, buried hill and inside-hill, Well Yaha10 and Well Zhongshen 5 are taken as examples for them respectively to ellucidate the characteristics of inter-platform gypsodolomite reservoir. The interval between 6 150–6 250 m in the Middle Cambrian Avatage Formation of Well Yaha 10 is buried-hill gypsodolomite reservoir, made up of gypsum concretion-bearing dolomicrite, it mainly contains gypsum casted mold pores, and intra-breccia pores in gypsum dissolved breccia (Fig. 5f, 5g), with a porosity between 1.50%–9.27%, 4.20% on average. CT scanning analysis of 25 mm mud-fine crystal dolostone samples shows isolated pore distribution, good differentiation between pore and throat, with a connected volume of 15.80%, and poor communication between pores and throats (Fig. 6e, 6f). The Middle Cambrian Avatage Formation, Shayilike Formation and Lower Cambrian Wusonggeer Formation in Well Zhongshen 5 all have inside-hill gypsodolomite reservoir,
which is gypsum-bearing or gypso-fine crystal dolomite with micro-pore between dolomicrite and fine crystal dolomite as main pore type (Fig. 5h, 5i). The pores, a product of dissolution of gypsum or non-dolomitization material, usually are less than 10μm in size, with a plane porosity between 1.5% –5.0%. CT scanning analysis of 3.5 mm dolomicrite and fine crystal dolomite samples show that the maximum, mean, and minimum radius of the pores are 4.68 μm, 1.86 μm, and 0.38 μm respectively; while the maximum, mean, and minimum radius of the throats are 3.22 μm, 1.31 μm, and 0.38 μm respectively, with unobvious differentiation between the pores and throats. But the connected volume accounts for 78.80%, indicating good connectivity between the pores and throats (Fig. 6g, 6h).
3. 3.1.
Genesis of dolomitic reservoir Material basis
Reef shoal sedimentary facies and gypsodolomite are the material foundation for the reservoir development. Regardless of carbonate reef and shoal inter-platform margin or carbonate mound-shoal in platform, porous sediments deposited in relatively high energy hydrodynamic environment act as the primary material basis, in which the primary pores could be intergranular pores in the carbonate granular shoal or carbonate framework or celom pores, that are not only good reservoir space, but also the migration paths for fluid later. In addition, where the reef and shoal developed would see the pleomorphic mound buildups, when the sea level dropped, the topography highs would be prone to leaching alteration from meteoric freshwater. In the gypsodolomite transitional belt of evaporative tidal
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flat environment, the gypsum often occurs granularly or in thin layers interbedded with dolomicrite. In the meteoric freshwater environment, the gypsum is prone to dissolution; the granular gypsum would be dissolved into cast molds, and the dissolution of thin gypsum layers would lead to the collapse of the overlying strata, resulting in the development of collapse breccia. Therefore, in the inter- and upper- flat in the evaporation environment, the precipitation of gypsum not only leads to dolomitization in Sabkha, but also lays the material foundation for the subsequent dissolution. 3.2. 3.2.1.
Porosity development and preservation Type and genesis of dolomite
The genesis of dolomite has been a research hotspot for years, multiple models[56] of dolomitization have been proposed by researchers, but there is a consensus that dolomitization can be divided into two stages: (1) penecontemporaneous-shallow burial stage, (2) middle-late burial stage. Penecontemporaneous dolomitization in Sabkha and refluxing seepage dolomitization in shallow burial stage are associated with evaporation environment, the short time and fast speed of the dolomitization result in finer crystal and preservation of primary rock structure of the dolomite; the dolomitization in middle-late burial stage is relatively slow, the dolomite formed in this stage is characterized by bigger crystal size, often above the level of fine grain, with partially remained structure of primary rock[7]. Regardless of outcrops or underground formations, the dolomite in the Lower Cambrian of Tarim Basin is mostly dolomicrite and fine crystal dolomite, algal mound dolomite and granular dolomite, with the structure of primary rock persevered, the granular composition of which mainly constitutes of mud-crystal, fine-crystal dolomite (Fig. 5a, 5c, 5d, 5f, 5h), with a small amount of dolomicrite and fine crystal dolomite (Fig. 5e). The former, formed in penecontemporaneous-shallow burial stage, is related to the arid climate in early Cambrian, obviously controlled by the distribution of sedimentary facies, and large in scale; the latter was formed in the middle-late burial stage along the faults, with limited distribution. Multiple geochemical parameters further reveal that the lower Cambrian dolomite in Tarim Basin is mainly the product of Sabkha dolomitization and refluxing seepage dolomitization, and formed in penecontemporaneous-shallow burial stage, only a small amount of fine-medium crystal dolomite shows the characteristics of middle-late stage burial dolomitization. According to the cross plot of CaO versus MgO contents (Fig. 7a), the CaO and MgO content of dolomicrite and fine crystal dolomite and some dolomicrite are positive linear correlated, indicating impure lithology of dolomite and adulteration of mud, typical characteristic of dolomitization in Sabkha; while, in some fine-crystal dolomite, the negative linear correlation between CaO and MgO content indicates short period
and incomplete dolomitization, which is the characteristic of refluxing seepage in shallow burial stage. According to the frequency statistics (Fig. 7b) of the dolomite order degree, algal dolomite in gypsum-bearing strata has the lowest order degree, indicating rapid dolomitization process in the high-salinity seawater, also typical characteristics of refluxing seepage dolomitization; the relatively low order degree of dolomicrite also reflects swift dolomitization in penecontemporaneous-shallow burial stage; and the relatively high order degree of dolomicrite and fine crystal dolomite, psammitic dolomite and medium-fine granular dolomite indicates long period of dolomitization, characteristics of both shallow burial and deep burial stage. According to cross plot of δ18O versus δ13 C (Fig. 7c), most dolomite samples have isotope δ18O greater than seawater in Early-Middle Cambrian, indicating that these dolomite samples were mainly formed in the low-temperature environment and affected by meteoric water, therefore, it can be inferred that the Lower Cambrian dolomite was formed in penecontemporaneous-shallow burial environment. It can be seen from 87Sr/86Sr ratio characteristics (Fig. 7d) that 87Sr/86Sr ratio of dolomicrite is significantly higher than that of the Cambrian seawater average (approximately 0.709 0), indicating an invasion of Sr from the crust, a typical characteristic of dolomitization in Sabkha; while the relatively high 87Sr/86Sr ratio of algal dolomite in the gypsum-bearing strata reflects the feature of evaporative seawater; the 87Sr/86Sr ratio of dolomite in other lithofacies is slightly higher than 0.709 0, indicating the effect of meteoric freshwater in the diagenesis. Trace elements cross plot of Na versus Sr (Fig. 7e) and Fe versus Mn (Fig. 7f) show that the medium-fine crystalline dolomite has low Na, Sr, and high Mn content, reflecting obvious features of middle-late burial stage; other types of dolomites are generally characterized by medium-high content of Na, Sr, and medium-low content of Fe, Mn, which indicate the dolomitization occurred in penecontemporaneous-shallow burial stage, the fluid for dolomitization was mainly seawater and the dolomitization has been affected by meteoric freshwater; small amount of dolomite samples have low Sr, Na and high Fe, Mn content, indicating they have been altered by fluid in late burial stage (Fig. 7g). It can be seen from the cross plot of Th versus U, and Al2O3 versus Fe2O3 (Fig. 7h) the dolomicrite samples have high Th/U ratio and Al2O3, Fe2O3 content, reflecting they were formed in oxidizing environment; the medium-fine crystalline dolomite samples have low Th/U ratio and Al2O3, Fe2O3 content, reflecting they were formed in reducing burial environment; generally, the formation environment of other types of dolomite fall between oxidizing and reducing, reflecting characteristics of shallow burial. 3.2.2.
Pore types and genesis
Pores in carbonate rock can be divided into three types by formaton stage: (1) sedimentary primary pores; (2) selective fabric dissolution pores resulted from the unstable mineral
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Fig. 7.
Geochemical characteristics of Lower Cambrian dolomite in Tarim Basin (Different colors refer to different samples in (d)).
(aragonite, high magnesium calcite and the likes) dissolution in epidiagenesis environment; (3) non-fabric selective dissolution pores resulted from the unstable mineral (aragonite, high magnesium calcite and the likes) dissolution in late epidiagenesis environment, the three compose the bulk of carbonate reservoir space[89]. The pores in the Lower Cambrian
dolomite of Tarim Basin are mainly primary depositional pores and selective fabric dissolution pores, while dissolution pores formed in late epidiagenesis have been found in Yaha-Yingmaili burial hill. The algal framework residue pore in inter-platform internal reef and shoal reservoir in the Xiaoerbulake Formation of
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Sugetbulak Section is typical sedimentary primary pore, with the peripheral infilled by dolomite cement; dissolution pores and vugs with obvious fabric-selective dissolution feature, were formed by the meteoric water dissolution in early epidiagenesis. The reservoir in Well Fang 1, a representative of inter-platform shoal and reef reservoir, mainly contains algae residual pores, with part of them filled by gypsum and dolomitic cement, belonging to sedimentary primary pore. The Middle-Cambrian in Well Yaha 7X-1 and the Lower Cambrian in Well Kang 2, representatives of inter-platform shoal reservoir, mainly contain intergranular and intragranular pores, the former is sedimentary primary pore, and the latter is formed as a result of selective dissolution of fabric in early epidiagenesis meteoric freshwater. Inter-platform gypsodolomite reservoir within insider-unit mainly contains micro-pores, which may be sedimentary primary pores, or be formed by freshwater dissolution of gypsum and marl in epidiagenesis environment. The burial-hill type of gypsodolomite reservoir mainly contains gypsum casted mold and gypsum-dissolved breccia pore formed by dissolution of meteoric freshwater in late epidiagenesis. The intergranular and intragranular pores in the small amount of powder-fine crystal dolomite of Middle-Lower Cambrian, are mainly sourced from the inheritance and adjustment of preexisting pores before burial[7, 10], some are formed by burial dissolution. 3.2.3. Contribution of dolomitization to porosity preservation To find out the contribution of dolomitization to porosity preservation, the modeling experiment of the effect of mineral composition on dissolution intensity has been carried out. The micritic limestone, marl, bioclastic-bearing micritic limestone and fine-sized dolostone samples were used in the modeling experiment which was completed at the Key Laboratory of carbonate reservoirs in CNPC. The experiment conditions were: 2 mL/L acetic acid solution, open-flow system, surface erosion, and flow rate of 3 mL/min, a total of 13 temperature and pressure points were tested, and modeling time for each temperature and pressure point was 30 min. The experiment results show that the dissolution rate of lime stone is faster than that of dolomite, as the burial depth increases, dissolution rate of limestone and dolomite in acetic acid solution increase gradually and tend to converge (see Fig. 8). The modeling experiments show that: (1) in deep layer, both limestone and dolomite under the alteration of organic acid, TSR and hydrothermal fluid, can be dissolved to form pores; (2) but in epidiagenesis environment, pores can be formed by dissolution in limestone, while can hardly be formed in dolomite. According to genesis of various kinds of dolomites and pores, and experiment results, it is concluded that the Lower Cambrian dolomite reservoir has undergone three key processes (Fig. 9): (1) sedimentary primary pores in reef and shoal
facies and gypsodolomite flat sedimentary system are key for reservoir development, and because of the arid climate, and thus undeveloped cement, only a minor of dolomite cement in the algae framework peripheral, the sedimentary primary pores have been well preserved; (2) the non-dolomitized limestone, gypsum in the penecontemporaneous stage lie the material foundation for the development of dissolution pores and cavities in the epidiagenesis, and the dolomite formed by the penecontemporaneous dolomitization acts as solid rock framework, protecting the porosity; (3) the dissolution cracks and pores formed by dissolution in late epidiagenesis improve the physical property of the reservoir. It can be seen that the dolomitization itself does not necessarily form porosity, but is very important for the development and preservation of porosity[7].
4.
Scale and distribution of reservoir
According to the genesis of reservoir, the development scale of all types of above-mentioned reservoirs has been analyzed. Platform margin reef and shoal reservoir characterized by large scale and good physical properties, is a very important exploration target in Lower Cambrian of Tarim Basin. This set of reservoir is widespread in the west platform margin of north Tarim Basin, outcropping in the Xiaoerbulake and Sugetbulak sections[4, 11], forming weak-rimed platform margin, with considerable potential. The Lower Cambrian Xiaoerbulake Formation in Well Fang 1 is a representative of inter-platform reef and shoal reservoir, which is the extension of reef shoal reservoir from platform margin to inter-platform, with the shoal being the major reservoir. In general, the rimed platform margin reef and shoal reservoir features “small reef and large shoal”, while the inter-platform mound and shoal reservoir features “small reef and small shoal”[12], but Tarim Basin in the Early Cambrian was a gentle slope-weak rimed platform, because of the gentle slope, algal mounds and associated sand shoal debris belts
Fig. 8. Modeling experiment results of the effect of mineral composition on the intensity of dissolution (Ion concentration of limestone using Ca2+, ion concentration of dolomite using Ca2++Mg2+).
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Fig. 9.
Model of the formation process of dolomite reservoir in the Lower Cambrian of Tarim Basin.
were wide, slight sea level fluctuation would result in the large-scale migration of mound-shoal, thus forming the extensive distribution of mound-shoal in the platform. Therefore, the inter-platform mound-shoal reservoir revealed by Well Fang 1 has huge potential too. Currently, two of the three wells drilling through the Lower Cambrian have encountered this set of reservoir. The inter-platform reef and shoal reservoir represented by the Middle Cambrian in Well Yaha 7X-1 is mainly composed of psammitic dolomite or fine crystal dolomite with psammitic residual structure, and abundant in intergranular pores, intragranular dissolved pores, and dissolved pores and cavities. As Tarim Basin in Middle-Cambrian was more arid, and was rimed platform, with saline lacustrines in continuous distribution, thus biotic reef in platform was not developed, characterizing by “small reef and small shoal”[12], so this set of reservoir is limited in scale. Currently, this set of reservoir has been found in only two of the nine wells drilling through the Middle Cambrian. The inside-hill gypsodolomite reservoir represented by the Middle Cambrian in Well Zhongshen 5, contains micro-pores formed by the dissolution of marl and gypsum between dolomite crystals. The development of gypsodolomite is mainly controlled by climate and facies belt, and the transitional facies belt tends to be favorable for development of this kind of reservoir. The paleogeography and paleoclimate in EarlyMiddle Cambrian of Tarim Basin mean that this set of reservoir could be in large pieces, and vertical stacking, reaching large scale. The buried-hill reservoir represented by the Middle Cambrian in Well Yaha 10 has rich gypsum casted molds, and intra-granular pores in breccia, the primary rock, dolomicrite bearing gypsum nodule, controlled by unconformity, distributes in buried-hill area only.
5.
Conclusions
The Lower Cambrian dolomite reservoir in Tarim Basin mainly includes three types: platform margin reef and shoal, inter-platform mound-shoal, and platform internal gypsodolomite, and the inter-platform gypsodolomite reservoir can be subdivided into inside-hill and buried-hill; the reservoir rock types are algal reefs (mound) dolomite, psammitic dolomite (partial recrystallization, representing fine-middle dolomite) and gypsodolomite, in which algae reef dolomite reservoir mainly contain algal framework pores, psammitic dolomite reservoir mainly intergranular, intragranular (dissolved) pores and intergranular (dissolved) pores, and the gypsodolomite mainly casted molds and micro-pores. The sedimentary primary pore in reef and shoal, and gypsodolomite flat is key to the development of reservoir; the non-dolomitized marl and gypsum in the penecontemporaneous stage lay the material foundation for the development of dissolution pores and cavities in early epidiagenesis; and the dolomite formed by penecontemporaneous dolomitization constitutes solid rock framework, protecting the porosity. The platform margin reef and shoal reservoir developed in the Middle-Lower Cambrian, large in scale and good in physical property, is a practical exploration target, while the inter-platform reef and shoal reservoir in the Lower Cambrian and the inside-hill gypsodolomite reservoir in the Middle-Lower Cambrian also have considerable potential.
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RESEARCH PAPER
Characteristics, origin and distribution of dolomite reservoirs in Lower-Middle Cambrian, Tarim Basin, NW China SHEN Anjiang1, 2, ZHENG Jianfeng1, 2,*, CHEN Yongquan3, NI Xinfeng1, 2, HUANG Lili1 1. PetroChina Hangzhou Research Institute of Geology, Hangzhou 310023, China; 2. Key Laboratory of Carbonate Reservoir, CNPC, Hangzhou 310023, China; 3. PetroChina Tarim Oilfield Company, Korla 841000, China
Abstract: Based on cores, thin sections and drilling data of 18 wells and two outcrop profiles of the Lower-Middle Cambrian in the Tarim Basin, geochemical analysis of multi-parameters in micro-area (the micro-carbonate fabric which is formed under the same diagenetic event or has the same genesis) and reservoir dissolution modeling were carried out to find out the types, origin and distribution of Lower-Middle Cambrian dolomite reservoirs. There develop three types of dolomite reservoirs, margin reef-shoal reservoir, platform interior mound-shoal reservoir and platform interior gypsodolomite reservoir. The rock types include algae dolomite, grain dolomite, and gypsodolomite; the pore types in them include algae framework pores in algae dolomite; intergranular pores, intra-granular dissolved pores, inter-crystalline dissolved pores in grain dolomite and gypsum-dissolved pore gypsodolomite. The primary pores in sediments of reef-shoal facies and gypsodolomite flat are the key to the development of the dolomite reservoirs, some aragonite and calcite are the material basis of dissolution in early supergene stage, and the dolomite formed in penecontemporaneous dolomitization acts as strong rock skeleton to protect the primary pores from destructive effect in burial stage. The margin reef-shoal reservoirs, large in scale and good in physical properties, are the most practical exploration targets; furthermore, the platform interior reef-shoal reservoirs in Lower Cambrian Xiaoerbulake Formation and platform interior gypsodolomite reservoir in the Middle-Lower Cambrian have higher oil and gas potential. Key words: Tarim Basin; Lower-Middle Cambrian; dolomite reservoir; margin reef-shoal; platform interior reef-shoal; platform interior gypsodolomite
Introduction With the deepening of understanding on the Lower Cambrian Yuertusi Formation source in Tarim Basin[1], Well Zhongshen1 and Zhongshen5 have been drilled subsequently since 2012, in which, the Lower Cambrian interval between 6 439–6 458 m in Well Zhongshen1 was tested an open flow of 5.10–15.40 m3 oil and 1 731–10 301 m3 gas a day with 3 mm choke; and the lower Cambrian 6 597.63–6 785.00 m was tested an open flow of 30 281 m3 gas and 34.92 m3 water a day with 5 mm choke, which unveiled the prelude of oil and gas discovery below the Cambrian salt. The lower Cambrian 6 861–6 944 m in the sidetracked Well Zhongshen1C was tested an open flow of 158 545 m3 gas a day with 5 mm choke, marking a strategic breakthrough in the exploration of dolomitic reservoir below the Cambrian salt[2]. Although exploration breakthrough has been made in the exploration of Lower Cambrian dolomitic reservoir in the basin, poor understanding in its reservoir type, origin and
distribution still hinders play and target assessment. Based on cores, thin sections, and single well data from 18 drilled wells (Yaha 5, Yaha 10, Yaha 7X-1, Xinghuo 1, Yingmai 7, Yingtan 1, Tong 1, Fang 1, 4, and 6, Shutan 1, Batan 5, Kang 2, Mabei 1, Zhongshen 1, Zhongshen 5, Tacan 1) encountering the Lower Cambrian, geochemical analysis of multi-parameters in micro-area (the same type of diagenetic minerals formed in different diagenetic events) and reservoir dissolution modeling were carried out to find out the types, characteristics and origins of the Lower-Middle Cambrian dolomitic reservoirs intra-salt and below-salt, and the distribution of scale dolomitic reservoir, to provide a basis for play assessment and target prospecting.
1. 1.1.
Geology setting of reservoir development Stratigraphic sequence and features
Middle-Lower Cambrian in Tarim Basin can be divided into platform, slope and basinal sedimentary facies systems[3], in
Received date: 05 Aug. 2015; Revised date: 25 Mar. 2016. * Corresponding author. E-mail: [email protected] Foundation item: Supported by China National Science and Technology Major Project (2016ZX05004-002); PetroChina Science and Technology Major Project (2014E-32-02). Copyright © 2016, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
SHEN Anjiang et al. / Petroleum Exploration and Development, 2016, 43(3): 375–385
in which reservoirs mainly develop in the platform area, upwardly, the strata sequence and lithofacies in the Cambrian (Fig. 1) are as follows: the Yuertusi Formation distributing in gentle slope and in basinal zone, is mainly composed of gray-green, gray-black mudstone, intercalated with thin-bedded limestone and diatomite, with a total organic carbon value (TOC) of 1.00 %–9.43%, averaging 5.5%, and a thickness of about 28 m; the lower part of the Xiaoerbulake Formation is mainly composed of gray-black dolomicrite and powdered crystal dolomite, while the middle-upper part of the Xiaoerbulake Formation mainly contain brown gray-gray grid algae, algae psammitic dolomite, and a small portion of dolomicrite, with a thickness of about 204 m, in which the reservoir is mainly in platform margin and inter-platform reef and shoal sedimentary facies; the Wusongge’er Formation about 103 m thick is characterized by large sets of dark gray-dark gray argillaceous dolomite, gypsodolomite, and dolomitic gypsum interbedding with a small clip of psammitic dolomite formed in arid climate, in which the reservoirs are gypsodolomite and psammitic dolomite; in the about 237 m thick Shayilike Formation formed in the evaporation climatic background, the upper part of the Shayilike Formation mainly contains gray-brown, dark-gray limestone, the middle-lower part mainly gray to brown gypsum, intercalating with purple mudstone, argillaceous dolomite, gypsum dolomite and a minor of psammitic dolomite, the reservoir is mainly in gypsodolomite and psammitic dolomite; in the Avatage Formation about 326 m thick formed in evaporation climatic background, the upper Avatage Formation mainly contains gray brown, dark gray argillaceous dolomite, marl, intercalating with dolomitic gypsum and a minor of psammitic dolomite, the middle-lower part is mainly gray, light gray brown gypsodolomite and a minor of psammitic dolomite, and the reservoir is mainly in gypsodolomite and psammitic dolomite; the Qiulitage Formation about 722 m thick mainly contains gray, dark-gray psammitic dolomite, dolomicrite, intercalating with marl dolomite. 1.2.
Sedimentary facies and paleogeography
Based on quantitative well logging of texture components of carbonate and seismic identification technology for carbonate lithofacies, the lithofacies paleogeography of Tarim Basin in Early-middle Cambrian has been delineated. In Early Cambrian, the Tarim Basin was a gentle slope-weakly rimed platform on the whole, the development of paleocontinent located in southwest Tarim Basin controlled the type and trending of sedimentary facies. Formed in semi-arid climate setting, the platform was mainly comprised of turbidite tidal flat, gypsodolomite flat, inter-platform shoal, and in-platform depression, with the sedimentary belts mainly trending NW-SE; and there mainly developed slow slope-weakly rimed platform margin (Fig. 2), algal reef (mound), and granulocyte debris shoal in the north and east part of the platform. In Middle Cambrian, the basin was a rimed platform with much more arid climate than that of Early Cambrian, and was char-
Fig. 1.
Stratigraphic column of the Cambrian in Tarim Basin.
acterized by the development of four different gypsum lacustrine sedimentary systems from the centre outward sequentially, the gypsum dolomitic flat-(argillaceous) dolomitic flat-platform margin depositional system belt (Fig. 3), gypsum saline lacustrine and gypsum dolomitic flat formed in arid climate comprised the main body of the inter-platform depositional system.
2. Types and characteristics of dolomite reservoir There develop three types of reservoirs in the Middle-Lower Cambrian in Tarim Basin, platform margin reef flat, inter-platform mound-shoal, and inter-platform gypsodolomite, in which the inter-platform mound-shoal reservoir can be subdivided into two types, buried hill and inside-hill, the reservoir is obviously controlled by the distribution of sedimentary facies. 2.1.
Platform margin reef shoal reservoir
Limited by the cores from drilling well, the study takes platform margin reef and shoal reservoir in the Xiaoerbulake Formation of Sugetbulak section in Keping region as an example. According to the characteristics of lithology and tested natural gamma (Fig. 4), the Xiaoerbulake Formation of
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Fig. 2.
Fig. 3.
Lithofacies and paleogeographic map of Tarim Basin in Early Cambrian.
Lithofacies and paleogeographic map of Tarim Basin in Middle Cambrian.
Sugetbulak section can be divided into two members, the upper and lower members, of which the upper member can be subdivided into the first, second, and third submembers. The Lower Member of Xiaoerbulake Formation mainly made up of thin-bedded gray black dolomicrite and fine crystal dolomite and pellet-fine crystal dolomite, is fairly stable in thickness and dense in lithology. The first submember of Upper Member fairly stable in thickness, mainly consists of gray-dark gray thin-bedded psammitic dolomite and fine crystal dolomite, gray algal-layered dolomite (intercalated with algae psam-
mitic dolomite lenticles) and algae psammitic dolomite, with small amount of pores in the algae psammitic dolomite. The second submember of the Upper Member of Xiaoerbulake Formation has thick psammitic dolomite, and pore and cavity increasing upward in the lower part, thick algal skeleton dolomite and widespread dissolved pores and cavities in the middle, a set of thick psammitic dolomite and stromatolitic dolomite, acicular dissolved pores increasing upward in the top. The third submember of the Upper Member of Xiaoerbulake Formation mainly consists of thin- to medium-layered
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Fig. 4.
Composite sedimentary column of the Xiaoerbulake Formation in Sugetbulak Section.
psammitic dolomite-bearing dolomicrite and fine crystal dolomite, medium-layered fine crystal dolomite, with pores in the top. Song Jinmin et al. described in detail the biological structure of the reef and shoal in the Upper Member of Xiaoerbulake Formation, and considered the reef and shoal was micro-organism genesis[4], this study also shows that the development of shoal and reef reservoir in the Lower Cambrian Xiaoerbulake Formation is related to fungi and algae. The second submember of the Upper Member of Xiaoerbulake Formation, about 47.5 m thick, is the main interval of reef and shoal, and also the major interval of reservoir (Fig. 4).
It is characterized by big shoal and point reef in the lower part, large reef in the middle and large shoal in the upper part. Pores are most developed in the algal sand debris shoal, algae skeleton reef in the upper part and algal stromatolite at the top. The pore types include algae skeleton, dissolved cavity, and intergranular pores. The algae skeleton pore mainly distributing in algal reef dolomite, is a typical kind of primary pore, partially filled by dolomite cement (see Fig. 5a). Dissolved pores, intergranular (dissolved) pores are mainly found in powdered crystal-fine crystal dolomite (particles phantom structure), algae psammitic dolomite, with selective fabric
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dissolution feature (Fig. 5b). Algae psammitic shoal dolomite has a porosity of 1.90%–9.39%, 5.50% on average, low energy beach between the reef has a porosity of 0.85%–4.10%, 1.76% on average, algal reef has a porosity between 1.32% and 10.55%, 4.81% on average. Quantitative CT analysis of 25 mm algal dolomite samples shows that the samples have various sizes of algal skeleton pores, unobvious differentiation between pores and throats, and 56.20% connected volume and good pore-throat connectivity (Fig. 6a, 6b). 2.2.
Platform internal mound-shoal reservoir
The Lower Cambrian inter-platform algal mound dolomitic reservoir in Well Fang 1 and the Middle Cambrian platform psammitic shoal dolomitic reservoir in Well Yaha 7X-1 are taken as examples to illustrate the characteristics of inter-platform mound-shoal reservoir. The 4 598.2–4 606.8 m in Well Fang1 of the Lower Cam-
brian Xiaoerbulake Formation is algal mound dolomitic reservoir, in which pores include mainly algal skeleton residue pores (Fig. 5c), and a small amount of gypsum-dissolved vugs. According to 27 samples analyzed, it has a porosity between 0.76%–3.57%, 1.55% on average, in which, samples with the porosity between 2.50%–3.57% account for 15.38%, samples with the porosity of 1.50%–2.5%, 23.38%, samples with porosity of less than 1.5%, 61.24%. Compared with platform margin algal reef reservoir in the Xiaoerbulake Formation in Sugaitebulaike section, generally, the inter-platform algal mound has poorer physical properties. The 5 815–5 840 m interval in the Cambrian Avatage Formation of Well Yaha 7X-1 is psammitic dolomite reservoir, in which pores include intergranular pores and intragranular dissolved pores, a small amount of dissolved pores (Fig. 5d). The reservoir has a porosity between 0.85%–6.97%, 4.6% on average, samples with porosity between 2.5%–4.5% account
Fig. 5. Types and characteristics of dolomite reservoir in the Lower Cambrian, Tarim Basin. (a) Algal dolomite, rich in framework pores, and the peripheral filled by early dolomitic cement, blue cast thin section, Xiaoerbulake Formation, in the Sugetbulak section; polarized light; (b) Psammitic dolomite, rich in dissolved pores and vugs along the stratum, outcrop samples of Xiaoerbulake Formation, in the Sugetbulak section; (c) Algal reef (mound) dolomite, rich in algae framework pores, the peripheral filled by minor early dolomitic cement and gypsum, Well Fang 1, Xiaoerbulake Formation; 4 599.00 m, red cast thin section, polarized light; (d) Psammitic dolomite, rich in inter- and inter-granular dissolved pores, Well Yaha 7X-1, Avatage Formation, 5 832.00 m, blue cast thin section, polarized light; (e) Psammitic dolomite, intergranular dissolved pore, Well Kang 2, Xiaoerbulake Formation, 5 497.00 m, blue cast thin section, polarized light; (f) gypsum-bearing dolomicrite, casted molds formed by gypsum concretion dissolution, Well Yaha 10, Avatage Formation, 6 170.95 m, blue cast section, polarized light; (g) Gypsum-dissolved breccia, rich in inter-breccia pore, Well Yaha 10, Avatage Formation, 6 210.40 m, red cast thin section, polarized light; (h) Micro-pore in dolomicrite, Well Zhongshen 5, A Watage Formation, 6 197.70 m, blue cast thin section, polarized light; (i) Gypsum-bearing dolomicrite, rich in micro-pore, Well Zhongshen 5, Wusongge’er Formation, 6 600.00 m, scanning electron microscopy.
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Fig. 6. Pore and throat structure of the dolomitic reservoir in Lower Cambrian, Tarim Basin. (a) CT three-dimensional imaging photo of a sample taken from Sugetbulak outcrop, Xiaoerbulake Formation; (b) the sample is same as (a), CT-based analysis of pore and throat radius distribution; (c) CT three-dimensional imaging photo, Well Yaha 7X-1, Avatage Formation, 5 832.00 m; (d) the sample is same as (c), CT-based analysis of pore and throat radius distribution; (e) CT three-dimensional imaging photo, Well Yaha 10, the Avatage Formation, 6 170.95 m; (f) the sample is same as (e), CT-based analysis of pore throat radius distribution; (g) CT three-dimensional imaging photo, Well Zhongshen 5, Wusongge'er Formation, 6 600.00 m; (h) the sample is same as (g), CT-based analysis of pore and throat radius distribution.
for 21.43%, and samples with porosity below 4.5% account for 64.29%. CT scanning analysis of 25 mm psammitic dolomite samples shows relatively uniform distribution of pores and throats, with vertical cyclicity, quantitative analysis shows little differentiation between pores and throats, and fairly good connectivity (Fig. 6c, 6d). Compared with Well Fang 1, it is found that the inter-platform psammitic shoal reservoir has much better physical property than algal mound reservoir. In addition, this kind of reservoir is also frequently seen in the Xiaoerbulake Formation, for exmaple Well Kang 2 (Fig. 5e) etc. 2.3.
Inter-platform gypsodolomite reservoir
Inter-platform gypsodolomite reservoir can be subdivided into two types, buried hill and inside-hill, Well Yaha10 and Well Zhongshen 5 are taken as examples for them respectively to ellucidate the characteristics of inter-platform gypsodolomite reservoir. The interval between 6 150–6 250 m in the Middle Cambrian Avatage Formation of Well Yaha 10 is buried-hill gypsodolomite reservoir, made up of gypsum concretion-bearing dolomicrite, it mainly contains gypsum casted mold pores, and intra-breccia pores in gypsum dissolved breccia (Fig. 5f, 5g), with a porosity between 1.50%–9.27%, 4.20% on average. CT scanning analysis of 25 mm mud-fine crystal dolostone samples shows isolated pore distribution, good differentiation between pore and throat, with a connected volume of 15.80%, and poor communication between pores and throats (Fig. 6e, 6f). The Middle Cambrian Avatage Formation, Shayilike Formation and Lower Cambrian Wusonggeer Formation in Well Zhongshen 5 all have inside-hill gypsodolomite reservoir,
which is gypsum-bearing or gypso-fine crystal dolomite with micro-pore between dolomicrite and fine crystal dolomite as main pore type (Fig. 5h, 5i). The pores, a product of dissolution of gypsum or non-dolomitization material, usually are less than 10μm in size, with a plane porosity between 1.5% –5.0%. CT scanning analysis of 3.5 mm dolomicrite and fine crystal dolomite samples show that the maximum, mean, and minimum radius of the pores are 4.68 μm, 1.86 μm, and 0.38 μm respectively; while the maximum, mean, and minimum radius of the throats are 3.22 μm, 1.31 μm, and 0.38 μm respectively, with unobvious differentiation between the pores and throats. But the connected volume accounts for 78.80%, indicating good connectivity between the pores and throats (Fig. 6g, 6h).
3. 3.1.
Genesis of dolomitic reservoir Material basis
Reef shoal sedimentary facies and gypsodolomite are the material foundation for the reservoir development. Regardless of carbonate reef and shoal inter-platform margin or carbonate mound-shoal in platform, porous sediments deposited in relatively high energy hydrodynamic environment act as the primary material basis, in which the primary pores could be intergranular pores in the carbonate granular shoal or carbonate framework or celom pores, that are not only good reservoir space, but also the migration paths for fluid later. In addition, where the reef and shoal developed would see the pleomorphic mound buildups, when the sea level dropped, the topography highs would be prone to leaching alteration from meteoric freshwater. In the gypsodolomite transitional belt of evaporative tidal
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flat environment, the gypsum often occurs granularly or in thin layers interbedded with dolomicrite. In the meteoric freshwater environment, the gypsum is prone to dissolution; the granular gypsum would be dissolved into cast molds, and the dissolution of thin gypsum layers would lead to the collapse of the overlying strata, resulting in the development of collapse breccia. Therefore, in the inter- and upper- flat in the evaporation environment, the precipitation of gypsum not only leads to dolomitization in Sabkha, but also lays the material foundation for the subsequent dissolution. 3.2. 3.2.1.
Porosity development and preservation Type and genesis of dolomite
The genesis of dolomite has been a research hotspot for years, multiple models[56] of dolomitization have been proposed by researchers, but there is a consensus that dolomitization can be divided into two stages: (1) penecontemporaneous-shallow burial stage, (2) middle-late burial stage. Penecontemporaneous dolomitization in Sabkha and refluxing seepage dolomitization in shallow burial stage are associated with evaporation environment, the short time and fast speed of the dolomitization result in finer crystal and preservation of primary rock structure of the dolomite; the dolomitization in middle-late burial stage is relatively slow, the dolomite formed in this stage is characterized by bigger crystal size, often above the level of fine grain, with partially remained structure of primary rock[7]. Regardless of outcrops or underground formations, the dolomite in the Lower Cambrian of Tarim Basin is mostly dolomicrite and fine crystal dolomite, algal mound dolomite and granular dolomite, with the structure of primary rock persevered, the granular composition of which mainly constitutes of mud-crystal, fine-crystal dolomite (Fig. 5a, 5c, 5d, 5f, 5h), with a small amount of dolomicrite and fine crystal dolomite (Fig. 5e). The former, formed in penecontemporaneous-shallow burial stage, is related to the arid climate in early Cambrian, obviously controlled by the distribution of sedimentary facies, and large in scale; the latter was formed in the middle-late burial stage along the faults, with limited distribution. Multiple geochemical parameters further reveal that the lower Cambrian dolomite in Tarim Basin is mainly the product of Sabkha dolomitization and refluxing seepage dolomitization, and formed in penecontemporaneous-shallow burial stage, only a small amount of fine-medium crystal dolomite shows the characteristics of middle-late stage burial dolomitization. According to the cross plot of CaO versus MgO contents (Fig. 7a), the CaO and MgO content of dolomicrite and fine crystal dolomite and some dolomicrite are positive linear correlated, indicating impure lithology of dolomite and adulteration of mud, typical characteristic of dolomitization in Sabkha; while, in some fine-crystal dolomite, the negative linear correlation between CaO and MgO content indicates short period
and incomplete dolomitization, which is the characteristic of refluxing seepage in shallow burial stage. According to the frequency statistics (Fig. 7b) of the dolomite order degree, algal dolomite in gypsum-bearing strata has the lowest order degree, indicating rapid dolomitization process in the high-salinity seawater, also typical characteristics of refluxing seepage dolomitization; the relatively low order degree of dolomicrite also reflects swift dolomitization in penecontemporaneous-shallow burial stage; and the relatively high order degree of dolomicrite and fine crystal dolomite, psammitic dolomite and medium-fine granular dolomite indicates long period of dolomitization, characteristics of both shallow burial and deep burial stage. According to cross plot of δ18O versus δ13 C (Fig. 7c), most dolomite samples have isotope δ18O greater than seawater in Early-Middle Cambrian, indicating that these dolomite samples were mainly formed in the low-temperature environment and affected by meteoric water, therefore, it can be inferred that the Lower Cambrian dolomite was formed in penecontemporaneous-shallow burial environment. It can be seen from 87Sr/86Sr ratio characteristics (Fig. 7d) that 87Sr/86Sr ratio of dolomicrite is significantly higher than that of the Cambrian seawater average (approximately 0.709 0), indicating an invasion of Sr from the crust, a typical characteristic of dolomitization in Sabkha; while the relatively high 87Sr/86Sr ratio of algal dolomite in the gypsum-bearing strata reflects the feature of evaporative seawater; the 87Sr/86Sr ratio of dolomite in other lithofacies is slightly higher than 0.709 0, indicating the effect of meteoric freshwater in the diagenesis. Trace elements cross plot of Na versus Sr (Fig. 7e) and Fe versus Mn (Fig. 7f) show that the medium-fine crystalline dolomite has low Na, Sr, and high Mn content, reflecting obvious features of middle-late burial stage; other types of dolomites are generally characterized by medium-high content of Na, Sr, and medium-low content of Fe, Mn, which indicate the dolomitization occurred in penecontemporaneous-shallow burial stage, the fluid for dolomitization was mainly seawater and the dolomitization has been affected by meteoric freshwater; small amount of dolomite samples have low Sr, Na and high Fe, Mn content, indicating they have been altered by fluid in late burial stage (Fig. 7g). It can be seen from the cross plot of Th versus U, and Al2O3 versus Fe2O3 (Fig. 7h) the dolomicrite samples have high Th/U ratio and Al2O3, Fe2O3 content, reflecting they were formed in oxidizing environment; the medium-fine crystalline dolomite samples have low Th/U ratio and Al2O3, Fe2O3 content, reflecting they were formed in reducing burial environment; generally, the formation environment of other types of dolomite fall between oxidizing and reducing, reflecting characteristics of shallow burial. 3.2.2.
Pore types and genesis
Pores in carbonate rock can be divided into three types by formaton stage: (1) sedimentary primary pores; (2) selective fabric dissolution pores resulted from the unstable mineral
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Fig. 7.
Geochemical characteristics of Lower Cambrian dolomite in Tarim Basin (Different colors refer to different samples in (d)).
(aragonite, high magnesium calcite and the likes) dissolution in epidiagenesis environment; (3) non-fabric selective dissolution pores resulted from the unstable mineral (aragonite, high magnesium calcite and the likes) dissolution in late epidiagenesis environment, the three compose the bulk of carbonate reservoir space[89]. The pores in the Lower Cambrian
dolomite of Tarim Basin are mainly primary depositional pores and selective fabric dissolution pores, while dissolution pores formed in late epidiagenesis have been found in Yaha-Yingmaili burial hill. The algal framework residue pore in inter-platform internal reef and shoal reservoir in the Xiaoerbulake Formation of
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Sugetbulak Section is typical sedimentary primary pore, with the peripheral infilled by dolomite cement; dissolution pores and vugs with obvious fabric-selective dissolution feature, were formed by the meteoric water dissolution in early epidiagenesis. The reservoir in Well Fang 1, a representative of inter-platform shoal and reef reservoir, mainly contains algae residual pores, with part of them filled by gypsum and dolomitic cement, belonging to sedimentary primary pore. The Middle-Cambrian in Well Yaha 7X-1 and the Lower Cambrian in Well Kang 2, representatives of inter-platform shoal reservoir, mainly contain intergranular and intragranular pores, the former is sedimentary primary pore, and the latter is formed as a result of selective dissolution of fabric in early epidiagenesis meteoric freshwater. Inter-platform gypsodolomite reservoir within insider-unit mainly contains micro-pores, which may be sedimentary primary pores, or be formed by freshwater dissolution of gypsum and marl in epidiagenesis environment. The burial-hill type of gypsodolomite reservoir mainly contains gypsum casted mold and gypsum-dissolved breccia pore formed by dissolution of meteoric freshwater in late epidiagenesis. The intergranular and intragranular pores in the small amount of powder-fine crystal dolomite of Middle-Lower Cambrian, are mainly sourced from the inheritance and adjustment of preexisting pores before burial[7, 10], some are formed by burial dissolution. 3.2.3. Contribution of dolomitization to porosity preservation To find out the contribution of dolomitization to porosity preservation, the modeling experiment of the effect of mineral composition on dissolution intensity has been carried out. The micritic limestone, marl, bioclastic-bearing micritic limestone and fine-sized dolostone samples were used in the modeling experiment which was completed at the Key Laboratory of carbonate reservoirs in CNPC. The experiment conditions were: 2 mL/L acetic acid solution, open-flow system, surface erosion, and flow rate of 3 mL/min, a total of 13 temperature and pressure points were tested, and modeling time for each temperature and pressure point was 30 min. The experiment results show that the dissolution rate of lime stone is faster than that of dolomite, as the burial depth increases, dissolution rate of limestone and dolomite in acetic acid solution increase gradually and tend to converge (see Fig. 8). The modeling experiments show that: (1) in deep layer, both limestone and dolomite under the alteration of organic acid, TSR and hydrothermal fluid, can be dissolved to form pores; (2) but in epidiagenesis environment, pores can be formed by dissolution in limestone, while can hardly be formed in dolomite. According to genesis of various kinds of dolomites and pores, and experiment results, it is concluded that the Lower Cambrian dolomite reservoir has undergone three key processes (Fig. 9): (1) sedimentary primary pores in reef and shoal
facies and gypsodolomite flat sedimentary system are key for reservoir development, and because of the arid climate, and thus undeveloped cement, only a minor of dolomite cement in the algae framework peripheral, the sedimentary primary pores have been well preserved; (2) the non-dolomitized limestone, gypsum in the penecontemporaneous stage lie the material foundation for the development of dissolution pores and cavities in the epidiagenesis, and the dolomite formed by the penecontemporaneous dolomitization acts as solid rock framework, protecting the porosity; (3) the dissolution cracks and pores formed by dissolution in late epidiagenesis improve the physical property of the reservoir. It can be seen that the dolomitization itself does not necessarily form porosity, but is very important for the development and preservation of porosity[7].
4.
Scale and distribution of reservoir
According to the genesis of reservoir, the development scale of all types of above-mentioned reservoirs has been analyzed. Platform margin reef and shoal reservoir characterized by large scale and good physical properties, is a very important exploration target in Lower Cambrian of Tarim Basin. This set of reservoir is widespread in the west platform margin of north Tarim Basin, outcropping in the Xiaoerbulake and Sugetbulak sections[4, 11], forming weak-rimed platform margin, with considerable potential. The Lower Cambrian Xiaoerbulake Formation in Well Fang 1 is a representative of inter-platform reef and shoal reservoir, which is the extension of reef shoal reservoir from platform margin to inter-platform, with the shoal being the major reservoir. In general, the rimed platform margin reef and shoal reservoir features “small reef and large shoal”, while the inter-platform mound and shoal reservoir features “small reef and small shoal”[12], but Tarim Basin in the Early Cambrian was a gentle slope-weak rimed platform, because of the gentle slope, algal mounds and associated sand shoal debris belts
Fig. 8. Modeling experiment results of the effect of mineral composition on the intensity of dissolution (Ion concentration of limestone using Ca2+, ion concentration of dolomite using Ca2++Mg2+).
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Fig. 9.
Model of the formation process of dolomite reservoir in the Lower Cambrian of Tarim Basin.
were wide, slight sea level fluctuation would result in the large-scale migration of mound-shoal, thus forming the extensive distribution of mound-shoal in the platform. Therefore, the inter-platform mound-shoal reservoir revealed by Well Fang 1 has huge potential too. Currently, two of the three wells drilling through the Lower Cambrian have encountered this set of reservoir. The inter-platform reef and shoal reservoir represented by the Middle Cambrian in Well Yaha 7X-1 is mainly composed of psammitic dolomite or fine crystal dolomite with psammitic residual structure, and abundant in intergranular pores, intragranular dissolved pores, and dissolved pores and cavities. As Tarim Basin in Middle-Cambrian was more arid, and was rimed platform, with saline lacustrines in continuous distribution, thus biotic reef in platform was not developed, characterizing by “small reef and small shoal”[12], so this set of reservoir is limited in scale. Currently, this set of reservoir has been found in only two of the nine wells drilling through the Middle Cambrian. The inside-hill gypsodolomite reservoir represented by the Middle Cambrian in Well Zhongshen 5, contains micro-pores formed by the dissolution of marl and gypsum between dolomite crystals. The development of gypsodolomite is mainly controlled by climate and facies belt, and the transitional facies belt tends to be favorable for development of this kind of reservoir. The paleogeography and paleoclimate in EarlyMiddle Cambrian of Tarim Basin mean that this set of reservoir could be in large pieces, and vertical stacking, reaching large scale. The buried-hill reservoir represented by the Middle Cambrian in Well Yaha 10 has rich gypsum casted molds, and intra-granular pores in breccia, the primary rock, dolomicrite bearing gypsum nodule, controlled by unconformity, distributes in buried-hill area only.
5.
Conclusions
The Lower Cambrian dolomite reservoir in Tarim Basin mainly includes three types: platform margin reef and shoal, inter-platform mound-shoal, and platform internal gypsodolomite, and the inter-platform gypsodolomite reservoir can be subdivided into inside-hill and buried-hill; the reservoir rock types are algal reefs (mound) dolomite, psammitic dolomite (partial recrystallization, representing fine-middle dolomite) and gypsodolomite, in which algae reef dolomite reservoir mainly contain algal framework pores, psammitic dolomite reservoir mainly intergranular, intragranular (dissolved) pores and intergranular (dissolved) pores, and the gypsodolomite mainly casted molds and micro-pores. The sedimentary primary pore in reef and shoal, and gypsodolomite flat is key to the development of reservoir; the non-dolomitized marl and gypsum in the penecontemporaneous stage lay the material foundation for the development of dissolution pores and cavities in early epidiagenesis; and the dolomite formed by penecontemporaneous dolomitization constitutes solid rock framework, protecting the porosity. The platform margin reef and shoal reservoir developed in the Middle-Lower Cambrian, large in scale and good in physical property, is a practical exploration target, while the inter-platform reef and shoal reservoir in the Lower Cambrian and the inside-hill gypsodolomite reservoir in the Middle-Lower Cambrian also have considerable potential.
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