Fluid inclusions in quartz: Implications for hydrocarbon charge, migration and reservoir diagenetic history of the Penyu Basin and Tenggol Arch, offshore Peninsular Malaysia

Abstract

A review of clastic sandstone reservoirs in the Penyu Basin and Tenggol Arch area, adjacent to the south-western flank of the Malay Basin revealed that most deep reservoirs are affected by diagenetic alteration of reservoir mineral components. Furthermore, fluid inclusions in quartz are seen at distinct stratigraphic reservoir levels. These inclusions occur in Oligocene reservoirs in Groups L and M and in Miocene reservoirs of Groups K and H. However, to-date no oil inclusions have been found in Groups I and J. There appear to be two distinct populations of fluid inclusions in the so-called 'oil quartzes': (i) oil inclusions in allochthonous, detrital quartz grains. These inclusions are thought to be of the primary type, formed when oil was incorporated in growing quartz crystals, and oil is seen encapsulated in 'loose' quartz grains. There is no evidence of destructive diagenesis or cementation in these relatively shallow (Miocene) host reservoirs, and oil migration is not confirmed by other indicators such as bitumen, which is often found in deeper carrier beds together with oil quartzes. The origin of these oil quartzes is somewhat controversial, and cannot be determined with certainty. They were probably shed into the basin from eroded granitic basement horsts and ridges.(ii) in situ oil inclusions in quartz cement. The occurrence of oil encapsulated in quartz cement (secondary or tertiary inclusions) indicates oil migration had preceded quartz cementation. So-far, oil-bearing inclusions attributed to fractures could not be confirmed with certainty. Assuming a relatively constant temperature gradient in the basin during the Miocene, quartz cementation will have started at a palaeo-depth of ca. 2000 m or at 105 0 C, and porosity was mostly destroyed by a depth of ca. 3000 m and 130 0 C. Occasionally overpressures are observed. According to this model, oil had migrated into and percolated within reservoirs during the Miocene, but became locked in closed pores as quartz cement invaded pore spaces under increasing overburden and temperature. Consequently, fluid inclusions in quartz for this model suggest that depths of greater than 3000 m below mud line (BML) are likely to encounter sandstones with deteriorating reservoir properties in the study area.

Overview of the structural framework and hydrocarbon plays in the Penyu Basin, offshore Peninsular Malaysia

“…Geochemical analyses have shown that thermally mature source rocks with mainly Type II/III kerogen may be present in the unpenetrated synrift section in those deep half-graben systems providing the hydrocarbon charge (Madon & Anuar, 1999). The heat flow is also markedly higher in some of the rift branches (Maga et al, 2015;Kessler & Jong, 2018) (Figure 21). Hydrocarbons could migrate along the fault conduits where they propagate upwards into the post-rift strata.…”

“…Further lithology and fluid prediction based on quantitative interpretation (QI) and seismic inversion studies incorporating well outcomes are recommended to help differentiate zones of better reservoir development. However, it is also noted that the intra-rift section in the graben area might have been affected by burial diagenesis resulted in deteriorating reservoir characters (Maga et al, 2015;Kessler & Jong, 2018).…”

Pre-Tertiary Basement Subcrops Beneath The Malay And Penyu Basins, Offshore Peninsular Malaysia: Their Recognition And Hydrocarbon Potential

“…Pre-Tertiary reservoirs appear to be of high risk, as diagenesis may have obliterated much of the porosity and, more so, permeability (e.g., Madon, 1994;Maga et al, 2015;Kessler & Jong, 2018). Therefore, the identification of enhanced porosity and permeability in the basement rocks due to intense fracturing is critical, while seismic recognition of fracture and fault orientations will be a key factor in finding new prospects.…”

“…From a petroleum geology standpoint, the pre-Tertiary reservoir potential is relatively high risk, as mineral transformation during late-stage diagenesis (e.g., Madon, 1994;Maga et al, 2015;Kessler & Jong, 2018) to early metamorphism may have obliterated much of the original porosity and, more so, permeability. In addition to tectonic fracturing associated with faults and shear zones, postdiagenetic and post-burial processes, including paleoweathering and karst formation in carbonates sequences, may also be important in enhancing the porosity of otherwise non-reservoir rock.…”

Radioactive apatite-rich “Hot Sands” of the Tenggol Arch: Stratigraphic curiosity or sub-seismic reservoir correlation tool?

“…Given that the source rocks on the Tenggol Arch are immature, both oil and gas must have been originated from either the Malay, Penyu or crust (White & Wing, 1978). Other tectonic models involve crustal extension over a hot spot (which is close to thinning of continental crust) (Hutchison, 1989;Khalid Ngah et al, 1996), extensional subsidence along a major left-lateral shear zone (Madon & Watts, 1998;Md Yazid et al, 2014;Maga et al, 2015;Kessler & Jong, 2018) and as a failed rift arm of a triple junction above a mantle hot spot (Tjia, 1999). Morley & Westaway (2006) proposed a geodynamic model of the Malay Basin, involving lower-crustal flow in response to post-rift sedimentation.…”