Deep basin hydrocarbon accumulation, also known as basin-centre or continous accumulation, have been recognised in the Rocky Mountain Laramide Basins of the centra-western US and in the Western Canadian Basin. Deep Basin Gas accumulations are unconventional in that they lie downdip of water-saturated rocks with no obvious impermeable barrier separating them. The porosity in such deep basin accumulations is almost entirely hydrocarbon-saturated. Deep basin accumulations are in hydraulic isolation and abnormally pressured with all porosity hydrocarbon-filled.
Fig 1. The difference between conventional hydrocarbon accumulations and deep basin hydrocarbon accumulations (source: Hills, 2001)
One of the key aspects of deep basin accumulation is that they are invariably abnormally pressured (Davis, 1984; Surdam et al, 1997). Abnormal pressures witness hydraulic isolation between the deep basin accumulations and overlying normally pressured water-bearing strata. Due to their hydraulic isolated nature, such pressure compartments are associated with fields that are not constrained by conventional structural closure or stratigraphic pinch-out (Al-Shaieb et al, 1994; Surdam, 1997).
Fig 2. Interpretation of abnormally pressured zones to locate deep basin hydrocarbon accumulations
The important points to recognised a deep basin gas play is that:
- Deep basin hydrocarbon accumulations appear to be associated with extensive coal rich source rocks capable of producing enormous volumes of hydrocarbons.
- Deep basin accumulations, all the rocks exhibits high resistivity.
- The key to commercial exploitation of deep basin accumulations lies not in intesecting gas, but rather in intersecting zones of enchanced porosity and permeability in the generally tight sandstones of deep basin settings.
- Deep basins are present where coarse-grained marine shoreline (beach barrier) sands occur.
- Natural fracturing of otherwise tight rocks can also help develop reservoir quality in the deep basin sands.
- The diagenetic history of sandstones is also critical to the preservation of reservoir quality in sandstones in deep basin settings.
- Thin permeable zones in deep basin accumulations can produce gas volumes in excess of that contained within the sweet spot itself
Last but not least, the exploration methodology for deep basin gas is considerably different than that for conventional. The search for commercial deep basin gas should focus on location depositional-structural-diagenetic sweet spots (DSDS) within anomalously pressured gas-saturated compartments, irrespective of conventional or stratigraphic closure.
Al-shaieb, Puckette, J.O., Abdalla, A.A, Ely P.B., 1994. Three levels of compartmentation within the overpressured interval of the Anadarko Basin. AAPG Memoir.
- R. Hills, J.G.G. Morton, D.S. Warner and R.K. Penney. 2001. Deep Basin Gas: A New Exploration Paradigm in the Nappamerri Trough, Cooper Basin, South Australia. Adelaide: Adelaide University
Davis. T.B., 1984. Subsurface pressure profiles in gas saturated basins. In: Masters, J.A. (ed) Elmworth-Case Study of a Deep Basin Gas Field: AAPG Memoir, 38, 1-35.
Spencer, C.W., 1989. Review of Characteristics of low permeability gas reservoirs in Western United States. AAPG Bulletin, 73, 613-29.
Surdam, R.C., Jaio, Z.S. Heasler, H.P., 1997. Anomalously pressured gas comparments in Cretaceous rocks of the Laramide Basins of Wyoming: A new class of hydrocaron accumulation. In: Surdam, R.C (ed) Seals, Traps, and the Petroleum System: AAPG Memoir, 67, 199-222.