Pore structure of the graptolite-derived OM in the Longmaxi Shale, southeastern Upper Yangtze Region, China

Highlights

• Pores in graptolite periderms are greatly associated with their fine structure.

• Pores are greatly developed in the discrete sapropel detritus.

• Graptolite periderms and porous sapropel detritus form a connected OM pore system.

• Provide new insights for OM pore heterogeneity study.

Abstract

The Lower Silurian Longmaxi Shale in the southeastern Upper Yangtze Region, which has been the main target for shale gas exploration and production in China, is black marine organic-rich shale and rich in graptolites. Graptolites, usually only periderms preserved in shales, are important organic component of the Longmaxi Shale. However, the pore structure of graptolite periderms and its contribution to gas storage has not yet been studied before. A combination of optical microscopy for identification and “mark” of graptolite and scanning electron microscope (SEM) for pore observations were conducted for the Longamxi Shale samples. Results show that pores are anisotropic developed in the Longmaxi graptolite periderms and greatly associated with their fine structure. Micrometer-sized fractures and spindle-shaped pores between cortical fibrils in the cortical bandage are greatly developed at section parallel to the bedding, while they are rare at section perpendicular to the bedding. Besides, numerous sapropel detritus rich in nanometer-sized pores are discretely distributed in the shale. Though graptolite periderms are low porosity from SEM image analysis, microfractures and elongated pores along the graptolite periderm wall may still make the graptolite an interconnected system. Together with the discrete porous sapropel detritus in shale, these graptolite-derived Organic Matter (OM) may form an interconnected organic pore system in the shale. The difference of pore development observed in graptolite periderms and sapropel detritus also give us new insight for the organic pore heterogeneity study. The OM composition, their fine structure and orientation in the rock may be important factors controlling OM pore development. The combination of identifying OM type under optical microscopy and pores observation under SEM for may be an effective method to study the OM pore development especially in shale that contain more OM.

Introduction

Recently high gas yields ranging from 3 × 10³ to 500 × 10³ m³/day in a single well have been achieved in the Lower Silurian Longmaxi Shale in the southeastern Upper Yangtze Region (Dai et al., 2014, Guo and Zhang, 2014). Particularly, the bottom section of the Longmaxi shale, which is black organic-rich shale and rich in graptolites (Fig. 1), is considered to be the main gas storage and producing layer (Guo and Liu, 2013, Zheng et al., 2013, Dai et al., 2014, Guo and Zhang, 2014). Luo et al. (2015) suggested that graptolites account for 20%–93% of the dispersed OM in the Longmaxi Formation based on numerous optical microscope and SEM observations. However, the pore structure of the rich developed graptolite-derived OM and its contribution to gas storage in the Longmaxi Shale has not been well studied before.

Graptolites are organic fossils that mainly occur in lower Palaeozoic (from Ordovician to Lower Devonian) marine sediments. Combined with their widespread distribution and rapid evolution, graptolites are of great importance in biostratigraphy and palaeoecology. Moreover, the reflectances of graptolite fragments have long been used to determine the maturation levels of the Pre-Devonian sediments (Kurylowicz et al., 1976, Goodarzi, 1984, Goodarzi and Norford, 1985, Bustin et al., 1989, Zhong and Qin, 1995) due to the lack of vitrinite (Zhong and Qin, 1995, Bustin et al., 1989). Graptolites were flattened in shale and only periderms were preserved as organic material (Bates et al., 1988, Bustin et al., 1989, Briggs et al., 1995). While Graptolites can be released from carbonates by dissolving the matrix.

It was initially believed that the graptolite periderm was (during life) constructed of a chitinous substance (Kozlowski, 1949) and subsequent work indicated a collagen-like characteristic of the fibrils composing the fusellar and cortical tissue of the periderm (Towe and Urbanek, 1972, Urbanek and Mierzejewski, 1986, Bates and Kirk, 1986). Bustin et al. (1989) used infrared spectroscopy to demonstrate that the graptolite periderm comprises principally an aromatic structure with aliphatic groups. With progressive maturation, the graptolite periderm underwent dealkylation with a loss of aliphatic C–H containing moieties (CH, CH₂ and CH₃). Then the aromatic system underwent structural transformation towards a more highly conjugated and condensed aromatic ring structure at higher level of maturation. The graptolite periderms are similar to Type II kerogen and exhibit a progressive decrease in hydrogen index with increasing maturation according to their hydrogen and oxygen indexes determined by Rock-Eval (Bustin et al., 1989). Briggs et al. (1995) concluded that the aliphatic material discovered in the periderm could not have been derived from original proteinaceous material such as collagen, and might have been derived by diagenetic replacement from algal cell walls. Gupta et al. (2006) emphasized the aliphatic composition of graptolite periderm reflects direct incorporation of lipids from the organism itself by in situ polymerization. Overall, the graptolite periderm might be aliphatic material and could be a good hydrocarbon-generating material at low maturation.

Histological studies of graptolite periderm structure can date to the middle of the 19th century (Barrande, 1850). However, results from optical microscopy were inconsistent and confusing prior to the publication of Kozlowski's (1949) classic monograph. Especially, Bulman (1970) summarized the well-known morphological studies of graptolites depending upon optical microscopy, establishing the relatively simple skeletal interpretation of an internal layer of fuselli and an outer layer of cortex.

Major advances in ultrastructural studies of graptolite have been made with the scanning electron microscopy (SEM) and Transmission Electron Microscope (TEM). Wetzel (1958) first used TEM technique, which was followed similarly by Kraatz (1964), to study the graptolite ultrastructure. Nevertheless, subsequent researchers had great difficulty in interpreting their results since the photomicrographs were poor quality. In the late 1970s, the work using SEM and TEM by Urbanek and Towe, 1974, Urbanek and Towe, 1975, Crowther and Rickards, 1977, Berry, 1978 and especially Crowther (1981) showed monographs which precisely defined the fine structure and elements of fusellum and cortex. The graptolite rhabdosome (skeleton of colony), was considered to consist of: (1) thecae (B in Fig. 2), which are the compartments once occupied by zooids; (2) apertures (C in Fig. 2), which are the external openings of each tube in the skeleton; (3) the common canal (D in Fig. 2), which is the cavity around the virgule into which the thecae open; (4) a virgella (E in Fig. 2), which is a continuous central structure that extends as a spine projecting from the thecae; and a periderm (see detail in Fig. 2), which is the substance composing the rhabdosome, with an inner layer (fusellar tissue) with growth bands and lines, and an outer layer (cortical tissue) of laminated material (Crowther, 1981). Each cortical bandage can be straight or variably sinuous, built from tightly packed unbranched fibrils and enveloped by a bounding sheet fabric (Crowther and Rickards, 1977, Crowther, 1981).

After that, the ultrastructure of graptolite internal tissues and graptolite-like pterobanchs were studied (Urbanek and Mierzejewski, 1984, Bates, 1996, Mierzejewski and Kulicki, 2001, Maletz et al., 2005). However, these work were based on irregular section surfaces either from broken surface or dissolved from the matrix.

Until the combination of SEM and Ar-ion-beam milling by Loucks et al. (2009), the namometer-sized organic-matter pores can be clearly observed. The importance of OM have been subsequently recognized as a significant component of gas storage and transport systems in shales (Ambrose et al., 2010, Passey et al., 2010, Sondergeld et al., 2010, Curtis et al., 2012a, Milliken et al., 2012, Milliken et al., 2013). However, the pore structure of graptolite periderms and its contribution to gas storage and flow has never been highly thought of before.

In this work, we selected several gas shale samples containing graptolites from the Longmaxi Shale in southeastern Upper Yangtze Region, China, to observe and mark graptolites on the polished surfaces under optical microscope. Then we studied in details the pore structure of the graptolite periderms in different directions under SEM.