Petroleum Geology of Cambodia
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Basin Map of Cambodia
TONLE SAP BASIN
Description
The Tonle Sap Basin covers an area of 23,800 km2. The basement is composed of deformed and metamorphosed Palaeozoic rocks, which crop out at the basin margins. Most of the basin is covered by Quaternary sediments, which overlie mainly terrigenous Mesozoic sediments expected to exceed 4000m in thickness based on gravity modeling. Salt bearing strata are present within the Cretaceous interval and may constitute good cap rocks for hydrocarbons (Vysotsky et al., 1994). By analogy with the Khorat syncline, a complex system of graben may be present and may be filled with Upper Carboniferous to Lower Permian and Upper Permian to Middle Triassic terrigenous-carbonate and volcanoclastic units, with a total thickness exceeding 1000m.
Source Rocks
Potential source rocks:
• Oil: Late Triassic – Jurassic ages (oil) transgressive marine strata.
• Gas: Upper Carboniferous – Middle Triassic organic rich lacustrine shales.
Maturation
The oil window has been modeled to occur occurs between 2000 and 2500 m (Vysotsky et al., 1994). This interval includes transgressive marine strata of Late Triassic – Jurassic ages, which may possibly be a source of liquid hydrocarbons. The Tonle Sap, Tuklith and Kleng depressions are considered to be particularly prospective areas. Upper Carboniferous to Middle Triassic rocks in the late catagenic zone may have generated gas, which has migrated into reservoirs, including both overlying Mesozoic formations and a carbonate unit of Late Carboniferous to Permian age.
Reservoir Rocks
Potential reservoirs:
• Mesozoic Formations
• Carbonate unit of Late Carboniferous to Permian age 
Traps and Seals
Potential seals:
• Mesozoic salt formations
• Claystones of Palaeozoic and Mesozoic age
Potential Traps:
• Inverted structures due to Indosinian uplifts and Himmalayan Orogeny
• Carbonate build-ups
• Stratigraphic pinch-outs
KHMER BASIN
Description
Offshore Cambodia is located in the southern part of the Eurasian plate, away from active centers of crustal subduction or sea floor spreading and Tertiary tectonic history heavily influenced by the collision with the Indian plate during Eocene times. The Khmer half graben experienced right lateral trans-tensional movement cause by extrusion of Indochina during Early Oligocene. The basin depocenter reaches depth of about 6000-7000 m.
Faulting in and around the Khmer trough is more complex and extensive then in the Pattani Basin. The area more subtly fault divided away from the Khmer Shelf structural high. This results in a series of various ridges separating narrow, fault bounded basin areas. Immediately adjacent to the Khmer Shelf two small and narrow north-south graben areas are present, the North Eastern Basin and the South Eastern Basin. To the west, a dominant north-south uplift region, the Khmer Ridge, is developed. Although a structurally high area, the Khmer Ridge also exhibits regions of regional sag elongated along its north-south axis.
Depositional environment is thought to be as follows:
• Late Miocene–Pliocene–Pleistocene: Shallow marine.
• Middle Miocene: Fluvial coastal plain.
• Early Miocene: Fluvial–lacustrine.
• Oligocene: Alluvial fan near basin.
Source Rocks
Late Oligocene and Early Miocene (late syn-rift) lacustrine shales and claystones are potentially the only effective source rocks in the basin. The distribution of source rocks and hence source potential (TOC) improves towards the northern end of the Khmer Basin and extends into the north-eastern basin where it is immature. Organic matter quality (HI) appears to be better in the south and east of the basin.
Although high average TOC is displayed at numerous stratigraphic levels, they are all considered immature apart from the lacustrine beds of the late syn-rift. According to Corelab the early syn-rift sequence has potential, if substantial thickness of source rock exists (in the depocentre). It would, however, be probably sufficiently mature to have expelled large volumes of dry gas.
Migration
Migration is believed to have been up dip through connected channels and faults. Seismic shows a high fault occurrence in the latter Tertiary so faults are considered as non barriers to the flow.
Reservoir Rocks
These are potentially developed in stacked sandstone sequence of Late Oligocene to Early Miocene age, which are anticipated to be fluvial channel sequence. Commercial reservoir range 12-20 m in thick, average thickness is 5.5 m. A typical discovery consist 8-20 m pay sand with on aggregate net thickness of 50 m over interval 700 m. Porosity range from 10 – 25%.
Plays
Proven plays are fault bounded stacked sands (channels) of Late Miocene -Oligocene. In order to penetrate as much accumulations as possible the wells should be deviated and at distances not higher than 200m from the fault plane.
Geology Map of Cambodia |
