Basement structure and basement structural history are two key parameters in any basin evaluation or regional geological analysis. A basement structure map provides a unique tool for interpretation of regional structural style and for correlation between basement structure and structure or stratigraphy in overlying sediments. It can also be used to explain relationships between basement structure and oil/gas distribution in the region. However, in many world-wide onshore/offshore regions, especially in areas where basement is believed to be deep, its configuration and actual depth can be very speculative. In those areas, there is usually a scarcity of basement-related hard data such as well penetrations or seismic data tied to well-controlled basement penetrations. As a result, attempts are often made to bridge this knowledge gap with generalizations or with unconstrained seismic interpretations. Unconstrained seismic can be a pitfall since “yesterday’s” interpretation of a “basement” reflector may be later shown, on the basis of newer and better acquisition or processing, to have been based on an intrasedimentary event (see Module #23-Basement Types for various definitions of “basement”). One way to avoid this problem is to incorporate or integrate quantitative magnetic depth estimates into an interpretation which should then be properly described as a magnetic basement interpretation.
Age, lithology, and stratigraphic position of rocks forming the basement are seldom well-known in the region studied. However, regional geologic comparative analysis can provide some useful interpretations. A basement structural interpretation should, if at all possible, be made by integration of magnetic, gravity, seismic, geology and other geophysical data to develop a series of model-tested regional maps. Some examples would be pre-Jurassic sediment isopachs, a basement isopach, or a lower crust isopach (thickness of oceanic/lower crust). The basement structure can also be integrated with gravity and seismic refraction data to help categorize crustal regimes: e.g., shield, platform, oceanic, etc.
Before beginning this module, you should have a firm concept of:
- How basement may be described or defined (Module #23)
- Anomaly patterns related to basement lithology and structure (Module #12)
- Regional structural framework interpretations based on qualitative gravity/magnetic interpretation (Module #32)