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, Malaysia
Corresponding author : [email protected]
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
SP
InterP
InterC
CF
CF
FP
InterP
InterP
Q
CF
K
Cb
F
I
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.