Search

Take the Cambrian Xiaoerblak Formation in the Keping (Kalpin) outcrop area as an example, a 28 km reservoir scale geological model was built based on description of 7 profiles, observation of more than 1000 thin sections, petrophysical analysis of 556 samples and many geochemical tests. The Xiaoerblak Formation, 158–178 m thick, is divided into three members and 5 submembers, and is composed of laminated microbialite dolomite (LMD), thrombolite dolomite (TD), foamy-stromatolite dolomite (FSD), oncolite dolomite (OD), grain dolomite (GD)/crystalline dolomite with grain ghost and micritic dolomite (MD)/argillaceous dolomite. The petrology features show that its sediment sequence is micro-organism layer – microbial mound/shoal – tidal flat in carbonate ramp background from bottom up. The reservoir has 5 types of pores, namely, framework pore, dissolved vug, intergranular and intragranular dissolved pore and intercrystalline dissolved pore, as main reservoir space. It is found that the development of pore has high lithofacies selectivity, FSD has the highest average porosity, TD, OD and GD come second. The reservoir is pore-vug reservoir with medium-high porosity and medium-low permeability. The dolomite of Xiaoerblak Formation was formed in para-syngenetic to early diagenetic stage through dolomitization caused by seawater. The reservoir development is jointly controlled by sedimentary facies, micro-organism type, high frequency sequence interface and early dolomitization. The class I and II reservoirs, with an average thickness of 41.2 m and average reservoir-stratum ratio of about 25.6%, have significant potential. It is predicted that the microbial mounds and shoals in the middle ramp around the ancient uplift are the favorable zones for reservoir development.

Deep dolomite in cratonic basins is an essential target for future natural gas exploration in China. Statistical analyses of major cratonic basins in China, such as the Sichuan, Tarim, and Ordos basins, indicate that the major types of deep dolomite (including granular dolomite, microbial dolomite, and crystalline dolomite) where fracture-vug and fracture-pore dominate the pore types. It is proposed that the favorable sedimentary microfacies is the base, whereas early dissolution is a necessary condition to form pores. Early dolomitization in penecontemporaneous and shallow burial periods is helpful to preserving the pores that developed during the penecontemporaneous period. Additionally, supergene karstification and structural fractures improve reservoir physical properties. It is pointed that sedimentary facies still control the referred high-quality deep dolomite reservoirs; they inherited more than being transformed in structure. The early sedimentary cycles, rather than depth, control their vertical distribution. Meanwhile, their lateral distribution is controlled by high-energy sedimentary facies belt rather than karstification. It is suggested that future exploration of high-quality deep dolomite reservoirs should target “both sides of an intra-platform rift” and low-middle parts of carbonate slopes. Keywords: Cratonic basin, Deep dolostone, Dolomite reservoir, Sichuan basin, Tarim Basin, Ordos Basin

The carbonate platform types and features of the platform margin belt plays an important role in controlling the reservoir formation; it also affects the relationship between reservoirs and caps. The Cambrian carbonate platform in the Tarim Basin underwent three evolutionary processes, namely, the Early Cambrian ramp platform, the Middle Cambrian edging evaporative platform, and the Late Cambrian edging platform; the northern platform margin was the deposition type, whereas the eastern platform margin was for fault control, additionally, the Lungu-Gucheng had evident sectional differences. The line from Wushi-Kashi-Maigaiti-Hetian to Minfeng of the southwestern Tarim Basin was an ancient land in the Early Cambrian. Through evolution, the sea level raised the underwater lows in the Middle and Late Cambrian period that possibly developed it to platform edge deposits in the Late Cambrian. The carbonate platform margin of both steep slope and gentle slope formed different reservoir-seal assemblages. The Upper Cambrian aggradation-progradation platform margin reservoir in the steep slope of the eastern Lungu and Gucheng area was developed with good connectivity, its caprock had been always the key of the platform margin reservoir-seal assemblages. Therefore, the reef-beach located behind the platform margin belt near the seaside of the lagoon had favorable reservoirs; the reservoirs often overlaid carbonate caprock which formed good reservoir-seal assemblages. The platform margin belt in the gentle slope in the Well Yingmai 7-Well Yingmai 8, in west Tabei, was a dolomite reservoir for algal mound and reef-bank complex with caprock of middle-lower Cambrian dolomicrite, gypsum dolomite, and mud dolomite. Aforementioned dense layers' reservoir-seal assemblage was superior to that of the eastern Lungu and Gucheng that had better exploration prospects.

Based on the reservoir characteristics of individual wells, the combined main controlling factor of fracture-vug reservoir and reservoir space synthesized modeling was adopted to establish geological models for the Ordovician reservoirs in the northern slope of the Tazhong palaeouplift and the southern area of the Tabei slope. Geological models for six types of wells were established: (1) cavity + fracture-vug controlled by sequence boundary; (2) cavity + fracture-vug on the top of cavity controlled by fractures; (3) cavity in the surface layer + fracture-vug under the cavity controlled by the sequence boundary and faults; (4) single fracture-vug controlled by the sequence boundary and faults; (5) single vug controlled by deep hydrothermal fluids along faults and fractures; (6) single fracture controlled by faults and fractures. Two types of synthesized geological models were established under the tectonic background of cuesta, namely, platform type slope of the Tazhong area and the gentle slope of the Tabei area. Type 1 and 2 reservoirs are common in the Yijianfang-Yingshan Formation of the HA6-Xinken Region, and type 3 and 5 reservoirs are common in the Yingshan Formation of the ZG8-ZG43 Region. A comprehensive evaluation shows that the reservoir of cavity + fracture-vug on the top of cavity controlled by the sequence boundary is the best, followed by cavity + fracture-vug on the top of cavity controlled by faults. So the areas that meet configuration of cavities and fracture-vugs and single vug controlled by deep hydrothermal fluids along faults and fractures are important targets for exploration and development. Key words: carbonates, fracture-vug reservoir, geological model, main controlling factors, Ordovician, Tazhong-Tabei area