Effect of different lithological assemblages on shale reservoir properties in the Permian Longtan Formation, southeastern Sichuan Basin: Case study of Well X1

Abstract

Various types of marine-continental transitional facies are present in the gas-bearing shales of the southeastern Sichuan Basin. A review of the different lithological assemblages in these rocks is important for assessing the likely shale gas content and the development of the storage space. This study of the lithological assemblages of the Permian Longtan Formation in the southeastern Sichuan Basin at Well X1 used core observations, optical thin-section observations, Ar-ion polishing, scanning electron microscopy, and nitrogen adsorption tests to compare and analyze storage space types and pore structures in the shale to determine the sedimentary paleoenvironment, petromineralogy, and organic content. The marine-continental transitional facies in the study area were deposited in a warm climate that favored enrichment by organic matter. The kerogen is type II2-III (average vitrinite reflectance 2.66%), which is within the favorable thermal maturity range for the presence of shale gas. The lithology mainly consists of shale, siltstone, and limestone (with bioclasts), as well as a coal seam. The lithological development divides the Longtan Formation into lower (swamp), middle (tidal flat/lagoon), and upper (delta) sub-members. From lower to upper divisions, the lithofacies evolved from silty shale to clay shale and then to shale intercalated with siltstone or calcareous layers. The proportions of intergranular and dissolution pores in the clay minerals decrease gradually from lower to upper sub-members, and pore size sizes also tend to decrease. Relatively large-diameter pores and microfractures occur in the inorganic matter in the lowest sub-member. Quartz and clay are the main constituents of the shale, respectively contributing to the specific surface area and specific pore volume of the reservoir space.

Fig 1. Geographical location, sampling location, and stratigraphic description of the Longtan Formation in the study area: (A) Sample location; (B) Well X1 in Longtan Formation (modified from [30]).

Fig 2

Thin section observations of lithological combinations in the Permian Longtan Formation, Well X1, southeastern Sichuan Basin: (A) Sample 3#: Argillaceous siltstone. The aggregation of clay minerals is mainly banded, and organic matter is clumpy and fine-banded, 5×10 (-); (B) Sample 7#: Shale. Organic matter is distributed along the bedding, 5×10 (-); (C) Sample 9#: Silty shale. Silty lamination develops, and organic matter is black-speckled, clumpy, and banded, 5×10 (-); (D) Sample 3#: Siltstone with mud. Organic matter is black, clumpy, and banded, 5×10 (-); (E) Bivalves and bryophytes. Bioturbation, horizontal bedding, nodular siderite; (F) Sample 4#: Shale with dolomite. Organic matter develops bedding, 10×10 (+); (G) Sample 6: Shale with siltstone. Bedding development, and organic matter develops bedding, 10×10 (-); (H) Sample 3#: Shale with bioclasts. Bioclasts are mainly spongy spicules scattered in rocks in needle-like and elliptical shapes, 5×10 (+); (I) Sample 8#: Carbonaceous mudstone. The mudstone is yellow-brown and is mainly an aggregation of microcrystalline and cryptocrystalline clay minerals. A few cracks are filled with organic matter, 10×10 (-); (J) Horizontal bedding; (K) Wave-ripple bedding; (L) Sample 9#: Silty shale, 5×10 (+); (M) Sample 14#: Bioclastic limestone. Bioclasts such as foraminifers and bryozoans were distributed elliptically, most of them metasomatized by calcite and siliceous minerals. A small number of ostracoda fragments were found scattered in the shape of tiny crescents, 5×10 (+); (N) Sample 6#: Mudstone. Siderite is nodular, spherical, with an inner radial structure, and overall characteristics of orthogonal extinction; and in sizes from 0.1–1.0 mm, 10×10 (+); (O) Sample 9#: Argillaceous limestone. Siderite mainly consists of fine microcrystalline granular and clumpy aggregates, 5×10 (+); (P) Lenticular bedding; (Q) Veined bedding and cross-bedding.

Fig 3. Triangular diagram of mineral composition of Permian Longtan Formation samples from southeastern Sichuan (Note: 25 of these data points from [30]).

Fig 4. Photomicrographs of typical organic material in Longtan Formation shale.

Fig 5. Distribution of TOC contents of the Longtan Formation samples: (A) Distribution of all TOC contents; (B) Distribution of TOC in different sections of the Lower, Middle, and Upper Longtan members (data from this study and [13, 15, 30]).

Fig 6. Distribution characteristics of microscopic reservoir space in shale samples of the Longtan Formation.

(A) Sample 8#: Interlayer fractures developing in lamellar clay minerals (×4300); (B) Sample 2#: Framboidal pyrite aggregations showing parts of the intercrystal pore filled with clay minerals (×1800); (C) Sample 5#: Irregular dissolution pores in siderite (confirmed by energy spectrum) (×3500); (D) Sample 4#: Interior of crustal organic matter filled with micro-granular and framboidal pyrite, as well as clay minerals (×1200); (E) Enlarged view of Sample 4# in Fig 9D, showing the development of little pores (×50000); (F) Sample 9#: Interstitial organic matter interbedded with clay minerals and siderite (720×); (G) Enlarged view of Sample 9# in Fig 9F, with pores and fractures developed (×5200); (H) Sample 12#: The sample structure is tight, clastic, interbedded with clay minerals and organic matter, with fragmentary or interstitial organic matter (×70); (I) Enlarged view of Sample 12# in Fig 9H. Irregular pores and microfractures developed in fragmentary organic matter (×4700); (J) Sample 9: Microfractures (×1500), Sd: siderite, Chl: chlorite, Kln: kaolinite, Qtz: quartz; (K) Sample 12#: Micro-granular quartz in inlying contact interbedded with lamellar kaolinite aggregations; intergranular pores and fractures developed (×1300); (L) Sample 14: No pores observed in banded organic matter, but shrinkage fractures developed at edges of organic matter (×40000).

Fig 7. Distributions of porosity, specific surface area, and specific pore volume of different samples.

Fig 8. Isotherm adsorption and desorption curves for different samples using low-temperature nitrogen.

Fig 9. Pore types and structure characteristic parameters in different shales by SEM and Image-pro Plus.

Aperture distribution histogram of all pores in (A) Sample 1# from Upper Longtan member; (B) Sample 6# from Middle Longtan member; (C) Sample 13# from Lower Longtan member. Distribution histogram of specific pore volume and specific surface area for (D) Sample 1# from Upper Longtan member; (E) Sample 6# from Middle Longtan member; (F) Sample 13# from Lower Longtan member.

Fig 10. Relationships between different mineral components (quartz, clay minerals) and pore volume, specific surface area, and microfracture development ratio.

(A) Relationship between quartz and specific surface area; (B) Relationship between quartz and specific pore volume; (C) Relationship between clay minerals and specific surface area; (D) Relationship between clay minerals and specific pore volume; (E) An irregular pore and microcrack in microgranular quartz (×3000); (F) Particle margin fractures in microcrystalline quartz (siliceous) (×3500); (G) Correlation between microfracture development ratio and brittleness index; (H) Interbedded fracture in lamellar andreattite aggregation (×10000); (I) Interlayer pores in alternately distributed lamellar andreattite and chlorite aggregation (×6500).