Basement Reservoirs - A Review of their Geological and Production Characteristics

Introduction This presentation describes the geological characteristics, technical issues and development challenges associated with hydrocarbon exploitation in crystalline basement formations. The term 'basement' here refers to crystalline formations ranging from intrusive and extrusive magmatic bodies (especially granites) to the family of low to medium grade metamorphic rocks. Hydrocarbons have been under production from these types of rocks around the world for many decades (see www.geoscience.co.uk for a review) but since around 1990 there has been growing interest and exploration in these formations where matrix porosity is neglible and storage and production are dominated by the fracture system (a Type 1 reservoir of Nelson 2001). For example, the Arabian shield basement of Yemen and the Tertiary basement granites offshore Vietnam are two classic areas for this type of development, and significant production has been achieved. Other less publicised locations have also been investigated including prospects on the UK continental shelf. The presentation draws on reservoir characterisation and modelling projects in basement hydrocarbon fields around the world, as well as investigations of basement rocks for deep geothermal resource exploitation (EGS) and also for sub-surface disposal of radioactive waste. The objective is to provide a synthesis and consolidation of learnings from these investigations in terms of the geological character and controls on fracture development, fluid flow and ultimately reservoir quality for hydrocarbon exploitation. Basement Charging Mechanisms Most basement hydrocarbons are hosted in structural highs of varying but generally moderate to large elevation (hundreds or even 1000m +). The highs are formed by fault-controlled blocks, often in rift settings, or by palaeo-hills buried below sedimentary cover. A second type of setting comprises intrusive igneous bodies (plutons) within sedimentary sequences. In all cases an overlying seal formation is found at some level. Additionally, some of the overlying, or fault-juxtaposed, sedimentary units may also be charged so that a composite hydrocarbon play is developed. For example, sandstone 'washes' above top granite surfaces or carbonate formations which developed on or adjacent to the fault blocks. Most basement charging is thought to have taken place by lateral or up-dip migration from kitchen areas in nearby structural lows. The migration route is through higher permeability sedimentary units, active faults and perhaps also at the basement / cover interface, thence into the basement itself. One example, from the basement fields West of Shetland on the UK continental shelf, is shown in Figure 1. Other examples are found in the rift basins offshore Vietnam (CuuLong Basin) and onshore in Yemen (eg Sab'atayn Rift).

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