Burial Diagenesis of Coal-Bearing Mudrock and Its Relationship to The Evolution of Pore Types and Abundance

Rosmalia Dita NUGRAHENI*, CHOW Weng Sum*, ABDUL HADI bin A. Rahman*, *Geoscience and Petroleum Engineering Department

Universiti Teknologi PETRONAS, Tronoh, 31750, Perak, MalaysiaCorresponding author : rosmalia.dn@gmail.com

Keywords – mudrock, burial diagenesis, pore-types, pore-network, pore-distribution

Abstract- The coastal region of Sarawak is primarily covered by thick Neogene coal-bearing mudrocks. The mudrocks are composed of shale or mudstone, alternating with sandstone layers and locally intercalated with coal beds. Carbonaceous material in the mudrocks in the form of maceral vitrinite is substantially important to generate gas. The study of pore-types and networks in mudrocks related to burial diagenesis provides the framework on how gas can be stored and flow through the pores. Nano- to micrometer-sized pores were observed in the matrix-related pore network of the mudrocks. These pores together with natural fractures form the flow path network that allows gas flow from the mudrock. The pore types associated with mineral particles are subdivided into interparticle and intraparticle. During burial, mudrocks undergo compaction that decreases substantial pore volume and it also causes thermal hydrocarbon maturation, where organic matter (OM) changes to kerogen. As such, the porosity and gas content is directly associated with the TOC content.

INTRODUCTION

The types of pores in the rocks are determined based on the relationships to particles as mentioned by Pittman, 1979. Pores formed by the arrangement of mineral grains can be interparticle (interP) and intraparticle (intraP). Organic matter also has intraparticle pores (intraP OM). Fracture also contribute to the rock porosity. Identification of pores, such as type, size, arrangement of pore, connectivity and wettability provides an insight to reservoir properties. Micro- to nano- scaled pore spectrum is easily recognized using scanning electron microscope (SEM), whilst macro-scaled pore can be seen under polarized microscope. The porosity of the mudrocks of the Balingian and Begrih Formation, Sarawak were studied in this project.

RESULTS and DISCUSSION

Typical interparticle pores are commonly found in the young or shallow-buried sediments of the Balingian and Begrih Formation with characteristics of well-connected and permeable layers. Subsequent overburden stress and diagenesis occurred during burial and closed the InterP pore spaces and plugged the pore throats. Ductile or plastically deformable grains (e.g. clay floccules, mica and organic matter) exacerbate the rate of porosity loss. The age of the sediments has a bearing on the volume of intraparticle pores (Fig. 1). Older sediments have less intraparticle pores as they are cemented by secondary minerals. The existence of fracture pores can have a significant effect on hydrocarbon production, because most of them are not completely cemented. During maturation of the organic matter, if it reaches a Ro level of approximately 0.6 or higher, intraP pores of organic matter begin to develop (Dow, 1977). This pore has an irregular, bubble-like, elliptical cross section and a length generally ranging from 5 to 750 nm. The pores appear to be isolated in 2D but are actually connected as displayed in 3D (Loucks et al., 2009). With a maturity of less than 0.6% Ro, the intraparticle OM pores are absent or extremely rare. Studies showed that the Balingian mudrocks have a Ro of 0.3 to 0.7% and some of the samples have bubble-shaped intraparticle OM pores (intraP OM). Balingian shales predominantly have Type III kerogen. Generally, type III kerogen shows no development of intraparticle OM pores, even if a mudrock has a Ro > 0.8%. It can be inferred that the type of kerogen present controls the formation of intraparticle OM in thermally matured rocks.

ACKNOWLEDGEMENT

The authors would like to thank Sarawak Coal Resources Sdn. Bhd. for their contributions in providing data and gaining access to the coal mines.

REFERENCES

Dow, W.G., 1977, Kerogen studies and geological interpretations: Journal of Geochemical Exploration, v.7, p. 79-99

Loucks, R.G.R.M., Reed, S. C., Ruppel and D.M. Jarvie, 2009, Morphology, genesis and distribution of nan0meter-scale pores in siliceous mudstones of the Mississippian Barnett Shale: Journal of Sedimentary Research, v.79, p.848-861

Pittman, E. D., 1979, Porosity, diagenesis and productive capability of sandstone reservoirs, in P.A. Scholle and P.R. Schluger, eds., Aspects of diagenesis: SEPM Special Publication 26, p. 159-173

Figure 1. Cartoon of pore types exist in the shale samples of the Balingian and Begrih Formation

InterC

SP

F

FP

I

K

Q

InterP

InterPInterP

CF

CF Cb

CF

Figure 2. left : Well-connected interparticle pores (InterP) in between quartz and clay floccules. Red lines represent possible hydrocarbon migration pathways; right : Typical of pyrite, sucrosic and framboids, create intercrystalline (InterC) and intraparticles (IntraP) respectively. Fracture pore

leads to the pore network.