Analysis reveals Longwangmiao porosity, permeability differences

Experiments

More than eight wells targeted to Longwangmiao formation recovered cores in the study area consisting of 10 outcrops in Sichuan basin. Investigation of reservoir characteristics of Longwangmiao formation used more than 149 dolomite samples from wells cores and outcrops, analyzing them in thin section for routine physical property as well as applying scanning electron microscope (SEM) and cathodoluminescence emission analysis.

Samples polished to 0.03 mm thick underwent finishing at State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University. Porosity and permeability characteristics of Longwangmiao reservoir were obtained by routine physical analysis of 80 samples. Subsequent testing occurred at State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology.

Reservoir petrology in the Longwangmiao formation, Lower Cambrian in the Northern Sichuan basin shows (a) Dolarenite dolomite, Longwangmiao formation, Well MS1; (b) Dolarenite dolomite, Longwangmiao formation, Shatan; (c) Powder crystalline dolomite with some intracrystalline pores, Longwangmiao formation, Well TX1; (d) Oolitic dolomite, Longwangmiao formation, Nanzheng; (e) Dolarenite dolomite, Longwangmiao formation, Well Z1; (f) Crystalline dolomite, Longwangmiao formation, Bashan (Fig. 2).

Lithological characteristics

The lithology of the Lower Cambrian Longwangmiao formation in central and northern Sichuan basin consists of dolarenite dolomite, oolitic dolomite, and crystalline dolomite. Dolarenite dolomite is one of the main reservoir rocks in the study area.

The northern Sichuan basin contains mostly meso-thin or lenticular deposits, with gray to light gray coloring (Fig. 2a), 50-65% dolarenite, 10-20% terrestrial clay and debris quartz, granular size of 0.16-0.40 mm, and poor sorting and psephicity (Fig. 2e). Intergranular dolomite cementing fill and selective dissolution of granular content and cements forms a small amount of intergranular dissolved pore with a surface porosity of 1-3% (Fig. 2b).

The dolarenite dolomite in the central Sichuan basin is medium-thick layered deposition, mainly gray-light gray color, dolarenite content of 50%~80%, granular size of 1-1.5 mm with good sorting and psephicity. Intragranular dissolved pores are developed due to strong dissolution (Fig. 3a) and have a needle-hole shape (Fig. 3b), surface porosity of 3-6%, locally up to 8-10%, and create the best reservoir rock in central Sichuan basin (Fig. 3d).

Reservoir petrology in the Longwangmiao formation, Lower Cambrian in the Central Sichuan basin shows (a) Oolitic dolomite, Longwangmiao formation, Well GS6; (b) Powder crystalline dolomite, Longwangmiao formation, Well MX10; (c) Powder crystalline dolomite, Longwangmiao formation, Well MX13; (d) Powder crystalline dolomite, Longwangmiao formation, Well MX8; (e) Dolarenite dolomite, Longwangmiao formation, Well GS17; (f) Oolitic dolomite, Longwangmiao formation, Well GS3 (Fig. 3).

Oolitic dolomite

Oolitic dolomite is mainly gray and the oolitic morphology is clearly visible on the core (Fig. 3f). Central Sichuan basin contains less than half as much oolitic dolomite as dolarenite dolomite, and its single-layer thickness is 2-8 cm interbedded with dolarenite dolomite. It is characterized by oolitic content of 50-80%, size of 0.2-0.8 mm, good sorting and psephicity, locally developed oolitic concentric layer, and mostly composed of fine crystalline dolomite with a right crystalline mosaic dolomite filling the inter-oolitic space. In contrast, oolitic dolomites in the northern Sichuan Basin are undeveloped, formed after dolomitization with morphology visible on the outcrops (Fig. 2f).

Crystalline dolomite

The crystalline dolomite reservoirs of the Longwangmiao formation in the study area are mainly composed of fine crystalline dolomite. The crystalline dolomites in the central Sichuan basin are medium-thin layered, with crystal size of 0.01-0.10 mm, and are mainly subidiomorphic (Fig. 3c). After dissolution, the intercrystalline pores and intercrystalline dissolved pores often develop, creating the good reservoir type found in the central Sichuan basin (Fig. 3e). In the Longwangmiao formation of the northern Sichuan basin, there are few fine crystalline dolomites, and the rocks are gray-light gray in color (Fig. 2f), mainly muddy-crystalline dolomite (Fig. 2c), and the reservoir is undeveloped.

Reservoir spaces

Analysis of field outcrops, well cores, thin sections, and SEM of the Longwangmiao formation in central and northern Sichuan basin divides the reservoir space into four types: intragranular dissolved pores, intergranular dissolved pores, intercrystalline dissolved pores, and fractures.

Intragranular dissolved pore space is the main reservoir space in the Longwangmiao formation and is mainly developed in the dolarenite dolomites and oolitic dolomites in the central and northern Sichuan basin. The intragranular dissolved pore is formed by dissolution in a carbonate granular environment during the deposition period, with strong dissolution keeping only the granular outline and leaving the granular spaces filled and semi-filled by dolomite and bitumen (Fig. 4a, Fig. 5a).

Reservoir spaces of the Longwangmiao formation, Lower Cambrian in the Northern Sichuan basin show (a) Intragranular dissolved pores, dolarenite dolomite, Longwangmiao formation, Bashan; (b) Intercrystalline pores, powder crystalline dolomite, Longwangmiao formation, Shatan; (c) Intercrystalline dissolved pores, powder crystalline dolomite, Longwangmiao formation, Fuchen; (d) the fracture is filled with some dolostones, Longwangmiao formation, Shatan; (e) intergranular dissolved pores, dolarenite dolomite, Longwangmiao formation, Well MS1; (f) Intercrystalline pores, powder crystalline dolomite, Longwangmiao formation, Well MS1 (Fig. 4).

Intergranular dissolved pore spaces formed by the strong dissolution of the dolomite cements in the sparry crystalline dolomite makeup one of the main reservoirs in the Lower Cambrian Longwangmiao formation. Pore diameter is often more than 1 to 2 times the size of dolomite crystal (0.50-2.00 mm) and partially filled by crystalline dolomite and bitumen. Pore edges are harbor-like with visible remaining cements. Large pores can form when dissolution is strong (Fig. 5b).

The intergranular pores in the central basin are extremely developed, often forming punctate-pore granular dolomite (Fig. 5d). The Longwangmiao formation in the northern Sichuan basin contains large amounts of terrigenous debris-insoluble residues, resulting in undeveloped dissolved pores and no basis as a reservoir (Fig. 4e).

Intercrystalline pores are mainly present in the crystalline dolomite and are developed between dolomite crystals with higher euhedral degree. The morphology is more regular (Fig. 5e) and the size of intercrystalline pores is generally restricted by crystalline size.19 The intercrystalline pore of the powder crystalline dolomite measures less than 0.01 mm, and the intercrystalline pore size of the fine-medium crystalline dolomite is 0.02-0.04 mm.

Reservoir spaces of the Longwangmiao formation, Lower Cambrian in the Central Sichuan basin show (a) Intragranular dissolved pores, dolarenite dolomite, Longwangmiao formation, Well MX204; (b) Intergranular dissolved pores, powder crystalline dolomite, Longwangmiao formation, Well MX202; (c) Intercrystalline pores, some filled with bitumen, finely crystalline dolomite, Longwangmiao formation, Well MX204; (d) Intergranular dissolved pores filled with little bitumen, oolitic dolomite, Longwangmiao formation, Well GS6; (e) Intercrystalline pores filled with some dolostones, finely crystalline dolomite, Longwangmiao formation, Well MS13; (f) the fracture is filled with some bitumen and dolostones, Longwangmiao formation, Well GS12 (Fig. 5).

Intercrystalline dissolved pores are formed by the migration of acidic water along the intercrystalline pores and dissolution of the bases of the intercrystalline pores, making the crystalline morphology of the euhedral crystalline dolomite irregular and creating harbor-like dissolution. The pore shapes are very irregular, with size of 0.03-0.10 mm.

Intercrystalline dissolved pore is one of the most developed reservoir space types in Longwangmiao formation. The intercrystalline pores and intercrystalline dissolved pores in the central Sichuan basin are often filled with dolomite and bitumen (Fig. 5c). In the northern Sichuan basin, intercrystalline pores are not developed (Fig. 4f) and didn’t suffer from strong dissolution, leading to undeveloped intercrystalline dissolved pores (Fig. 4c). The intercrystalline pores in some muddy-powder crystalline dolomite rocks are also filled with silk illite and booklet kaolinite and other clay minerals (Fig. 4b).

The fractures of Longwangmiao formation in central and northern Sichuan basin are well developed. The fractures in central Sichuan basin are structural fractures and dissolution fractures. The dissolution fractures are generally fully filled with dolomite and bitumen (Fig. 5f). In northern Sichuan basin, the structural fractures are more common and mostly filled by calcite or dolomite, but the dissolution fractures are not developed (Fig. 4d).

Petrophysical properties

The porosity and permeability tests on samples from the Longwangmiao formation in the central and northern Sichuan basin showed porosity of the 82 samples in central Sichuan basin of 1.47-11.35%, with 43.21% having porosity of 4-6%. Average porosity measured 5.32%. Permeability was 2.63 × 10-4 to 45.27 md, with 33.33% of sample permeability between 0.1 and 1.0 md. Average permeability measured 30.4 md (Fig. 6a).

The porosity of 67 samples from northern Sichuan basin was 2.07-6.01%, with 57.1% having porosity of 2-3%. Average porosity equaled 3.19%. Permeability was 6.22 × 10-5 to 2.24 md, with 64.5% of the sample having permeability of 10-5 to 10-3 md and average permeability measuring 0.05 md (Fig. 6b).

The data show that permeability and porosity of the reservoirs of Longwangmiao formation in the central Sichuan basin are positively related. Pore type is mainly intergranular dissolved pores and intercystalline dissolved pores. Porosity and permeability in the northern Sichuan basin are lower than those in the central Sichuan basin and porosity and permeability in the reservoir are mostly low. Sediments in Longwangmiao formation in this area contain more terrigenous clay mud and undissolved terrestrial clastic quartz, which lead to low original porosity and are not favorable to later diagenesis. Reservoir characteristics of Longwangmiao formation in northern Sichuan basin are worse than those in central Sichuan basin.

Main controlling factors

Depositional environment is a prerequisite for carbonate reservoir development. Sedimentary differentiation within the carbonate platform is controlled by the paleogeomorphology formed by tectonic activity, marine transgression-regression, and other factors. Sedimentation provides the material basis for later diagenetic transformation, only the pure carbonate layer with good purity, high thickness, and good primary porosity being favorable for later diagenetic transformation.20

During the early Cambrian Longwangmiao period, the whole Sichuan basin was in the carbonate platform depositional environment, the water body deepening west-to-east. Terrestrial debris and clay from Hannan paleocontinent deposit formed point-like granular shoal sedimentation, confining the later diagenetic formation and a relatively undeveloped reservoir area.

The central Sichuan basin is far away from the source of the Kangdian paleocontinent and is affected by the persistent Leshan-Longnvsi paleo-uplift. Along the eastern paleo-uplift near the sea, granular shoal sedimentation formed in a large belt-shaped area containing a very small amount of large terrestrial debris, making it favorable for reservoir development (Fig. 7).

By influence of the Caledonian movement in the early Cambrian Longwangmiao period, the central Sichuan basin was uplifted to the surface subjecting it to weathering leaching. The highest portion of Leshan-Longnvsi paleo-uplift and the surrounding area experienced widespread hypergenesis karstification.21

The well core shows well-developed beehive-like dissolved pores in the Longwangmiao formation, especially the elongated-orientation pores mainly formed by atmospheric freshwater dissolution along the formation during the tectonic erosion of Leshan-Longnvsi paleo-uplift.22 The Longwangmiao formation in the northern Sichuan basin is far from the Caledonian Leshan-Longnvsi paleo-uplift and did not experience hypergenesis karstification. The intergranular mud dolomite content is high and the primary porosity is relatively undeveloped and did not experience hypergenesis karstification, so dissolved pores were not widely formed in the northern Sichuan basin (Fig. 8).

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The author

Longbo Xu ([email protected]) is a Ph.D student in geological resources and geological engineering at Southwest Petroleum University, Chengdu, Sichuan Province, China.