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Reservoir Type and Forming Mechanism of Sinian-Ordovician Dolomite in China

Zhiliang He, Juntao Zhang, Jufang Feng, Yixiong Qian, Cunli Jiao, and Shoutao Peng
Petroleum Exploration and Production Research Institution of Sinopec, Beijing, China

1. Geological setting of Sinian to Ordovician dolomite in three craton basins in China Dolomite is widely developed in three craton basins from Sinian to Ordovician in China, which is time equivalent to the global dolomite events. Significant hydrocarbon discovery has been made in Sinian-Ordovician dolomite. High quality dolomite reservoirs are the necessity of commercial hydrocarbon discoveries.

From Sinian to Ordovician, sedimentation in Tarim Basin is dominated by platform carbonates, which shows a sequence of transgressive-regressive-transgressive series. Algae dolomite of Qigebulake Formation in the Upper Sinian experienced short exposure and surface weathering. Dark grey limestone intercalated by mudstone of the Yuertusi Formation in Lower Cambrian become very important source rock in the basin. Dolomite of Xiaoerbulake Formation is deposited in the subsequent regressive event. Extensive evaporites were deposited at the top of Lower Cambrian and Middle Cambrian. The upper part of Lower Ordovician is a transition between dolomite and limestone, and at the top of Lower Ordovician it is dominated by limestone.

The depositional environment during Late Sinian in Sichuan basin is interpreted to be tidal flat-lagoon, mostly restricted platform with some beach facies inside the platform. Common lithofacies in the Dengying Formation include algae dolomite, crystalline dolomite. During Cambrian, the depositional environment in Sichuan basin is described as a sequence of beach- tidal flat- restricted (open) platform- platform margin- platform ramp- basin from west to east. Water body became shallower and climate got drier in the early and middle Cambrian, and until late Cambrian it was an open platform- restricted platform in this area. Rock types in the southwest and middle are mainly grain dolomite, and are composed of silty dolomite and intraclast dolomite in the southeast Sichuan. Upper and middle Cambrian is mainly crystalline dolomite and coated grain dolomite in the southwest and middle Sichuan area, while it is a set of dolomite and muddy limestone in the southeast Sichuan. The lithofacies of Late Cambrian is a set of crystalline dolomite and coated grain dolomite. Early Ordovician dolomite in Sichuan basin is deposited in mixing tidal flat and open platform.

In Paleozoic, paleogeography of Erdos basin evolved from coastal deposition in Early Cambrian to restricted- open platform deposition in Middle and Late Cambrian. This is followed by restricted- evaporite platform sedimentation in Early and Middle Ordovician, and then deposition on the rim of platform in an epicontinental sea. Most of the deposition in middle and east of Erdos is shallow water carbonates. Deposition in Early and Middle Cambrian is mostly muddy and sandy dolomite. Lithofacies of Early Ordovician is basically medium- thick layered light grey to grey dolomite. Middle Ordovician Majiagou Formation shows a frequent change in lithfacies, and the rock types include dolomite containing gypsum, dolomite, gypsum, and limestone. Pingliang Formation and Beiguoshan Formation in Late Ordovician is dominated by limestone deposition.

2. Reservoir characteristics of Sinian- Ordovician dolomite in three craton basins in China

2.1 Dolomite reservoir in Tarim basin

Dolomite reservoir is developed in the upper of Lower Cambrian in the Keping area of Tarim basin. It shows obvious difference from the original dolomite. Dolomite experienced thermal alternation shows sility crystalline and anhedral features, displaying coated grain ghost. It can be predicted that the parent rock before dolomitization is coated grain limestone.

Lithofacies in the Yingshan formation in the Guchengxu paleohigh in Tarim basin is dominated by calcium dolomite, which is the product of incomplete dolomitization during burial phase. Reservoir space is mainly inter- crystalline, which is formed due to selective dissolution of inter- crystalline calcite. Dolomite with certain quantity of calcite is the basis of burial dissolution. Since only calcite is dissolved and form the vuggy pore space due to limitation of the fluid dissolving capacity.

The main lithofacies of the dolomite reservoir in Yakela area in the northern Tarim basin is fine- medium crystalline dolomite, with occasional occurrence of coated grain dolomite. It is a result of burial dolomitization and recrystallization. The reservoir is developed below unconformity, and pore spaces are dominated by inter- crystalline and fracture.

2.2 Dolomite reservoir in Sichuan basin

Algae dolomite is the dominant rock type in Leshan-Longnvsi paleo-high, and followed by crystalline and intraclasts dolomite. Dolomite reservoir in Ziyang area is mainly developed in the second and fourth Member of Dengying formation, in which the best reservoir quality developed near the erosion surface. Reservoir porosity type of the Dengying formation includes: channel, cavern, vug, and structural facture. The fractures are mainly structural genesis and have the characteristics of multiphase alternation.

Cambrian reservoir is distributed across the basin, and the dolomite reservoir shows features of low porosity and permeability. Second porosity such as inter-crystalline porosity, moldic and intra-crystalline porosity were observed in some lithofacies, and special vuggy dolomite was developed locally.

Lithofacies of middle Ordovician dolomite reservoir contain silty-fine crystalline dolomite, silty dolomite in central Sichuan area. Reservoir pore space is a combination of intra-particle, inter- particle, and inter- crystalline porosity. Occurrence of reservoirs is clearly controlled by original sedimentary environments, karstification and fracturing.

2.3 Dolomite reservoir in Erdos basin

Dolomite reservoir in Sanshanzi Formation of Late Cambrian and Zhixi Formation in Early Ordovician in the Erdos basin is medium- coarse dolomite, which is the product of highly dolomitization and recrystallization. Mineral components contain mainly dolomite, a small proportion of calcite, quartz, and so on. Primary sedimentological texture is rarely preserved. The order degree of dolomite is high, with the presence of zonal texture in the crystal, which result from high level of dolomitization. It is formed at deep burial depth. The pore are mainly inter- crystalline and irregular vuggy porosity.

The most important reservoir of Lower Paleozoic dolomite in the Erdos basin is the dolomite containing gypsum in the fifth Member of Majiagou Formation. The major mineral component is muddy- silty crystalline dolomite. Anhydrites occur mostly as round and sub-rounded nodules or planar/strip crystals, but they are usually replaced by calcite, quartz, and dolomite. Besides, a lot of sub-rounded pore space formed from dissolving gypsum nodules, planar, and cubic gypsum crystal, and they are filled with minerals to different levels. Muddy- silty crystalline dolomite reservoir has a low degree of order, indicating formation of dolomite during parasyngenetic dolomitization process.

3. Forming mechanism of Sinian to Ordovician dolomite reservoir in three craton basins in China

Lower Paleozoic dolomite reservoir in three craton basins in China occurred in many types , so the forming mechanisms are different. Previous research suggested many kinds of classification about dolomite reservoir according to different standard, but it still could not cover all the reservoir types. It is proposed to use lithofacies and fluid as two key factors to categorize the dolomite reservoirs, and then analysis about the forming mechanism of each reservoir type is carried out respectively. According to the classic classification of carbonates, lithofacies of dolomite is classified as coated grain dolomite, biogenic dolomite, crystalline dolomite, and dolomite containing specific or easily soluble components (anhydrite and calcite). According to the phase and characteristic of fluids, the categories are: meteoric and mixed water dissolution during parasyngenetic diagenesis, dissolution during burial, supergene dissolution by meteoric waters, and hydrothermal dissolution during special burial phase. Based on the actual geologic background of three basins, dolomite reservoirs are classified into the following categories to describe the forming mechanism.

3.1 Coated grain dolomite- parasyngenetic dissolution

This type of dolomite reservoir is dominated by coated grain dolomite. The pore of the reservoir is mainly intra- particle porosity or inter- particle porosity, which is formed from meteoric dissolution during parasyngenetic diagenesis. This type of reservoir scattered in the Paleozoic strata in three craton basins. For example, the Cambrian deposition in Sichuan basin is a good example.

3.2 Coated grain dolomite- hydrothermal dissolution during burial phase

The major rock type is coated grain dolomite in this kind of reservoir, and the primary sedimentary facies is beach facies. Reservoir space is mainly intra- particle or inter- particle porosity, which is formed from hydrothermal dissolution related to fractures during burial phase. One of the examples is the upper part of Xiaoerbulake Formation in Lower Cambrian in Tarim basin.

3.3 Crystalline dolomite- supergene dissolution by meteoric waters

The main rock type is crystalline dolomite, and the primary sedimentary facies is tidal flat. Primary reservoir space type is irregular dissolution vug formed due to karstification and superposition of following dissolution during burial phase. For example, the Qiulitage subgroup in Yakela area of Tarim basin.

3.4 Crystalline dolomite- hydrothermal dissolution during burial phase

In this kind of reservoir, the rock type is mainly crystalline dolomite and the primary sedimentary facies are usually tidal flat. Reservoir space type is dominated by intercrystalline porosity which is formed from dissolution during burial phase. The fourth Member of Majiagou Formation in Dingbei area of Erdos basin is a good example.

3.5 Dolomite containing easily soluble components- dissolution during burial phase

The major rock type is calcium dolomite in this type of dolomite reservoir, and the main sedimentary facies is tidal flat and platform. Pore space is basically inter- crystalline porosity, which is formed from selective dissolution during burial phase. A good example is the lower part of Yingshan Formation in the Guchengxu paleohigh of Tarim basin.

3.6 Dolomite containing easily soluble components- supergene dissolution by meteoric waters

The main rock type is dolomite containing anhydrite, and the primary sedimentary facies is evaporite tidal flat facies. Observed reservoir space is mainly moldic porosity from dissolution of gypsum. The origin of the porosity is selective supergene dissolution by meteoric waters, such as the fifth Member of Majiagou Formation in Erdos basin.

3.7 Biogenic dolomite- supergene dissolution by meteoric waters

In this kind of reservoir, the rock type is mainly algae dolomite and the primary sedimentary facies is tidal flat. The pore space is dominated by irregular dissolution vug, which is formed from supergene dissolution by meteoric waters. One of the examples is the Sinian Dengying Formation in Sichuan basin.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013