Paleogeography and prospect identification

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Critical elements of the petroleum system
Chapter	Sedimentary basin analysis
Author	John M. Armentrout
Link	Web page
Store	AAPG Store

Basin paleogeography is defined by picking an isochronous or coeval surface or interval and mapping the associated seismic facies and lithofacies. For example, the location of sandy lithofacies vs. clayey lithofacies or mounded vs. tabular seismic reflection configurations may delineate the position of shorelines or reefs. Mapping thickness of reservoir-quality rocks is also useful for establishing paleogeography; thick, linear, dip-oriented trends of sandstone may indicate paleochannel complexes.

Procedure

The list below outlines a suggested procedure for defining paleogeography and applying it to prospect identification.

1. Identify the stratigraphic interval of a single depositional phase that has potential for containing reservoir rocks, i.e., lowstand or highstand, using biostratigraphic markers and regional correlation surfaces.
2. Map depositional facies such as biofacies, net reservoir thickness (lithology or porosity), and seismic facies (i.e., clinoforms, parallel reflections, chaotic reflections within a single depositional phase).
3. Integrate the interpreted seismic facies and biostratigraphic data into a grid of stratigraphic well-log cross sections if wells are available.
4. Map the location of fields producing from reservoirs in the interval of interest with respect to net reservoir thickness; define the type of trap(s) each field contains.
5. Using the seismic interpretations and the geology of the fields mapped in step 4, interpret the deposition of reservoir, seal, and source facies and the formation of stratigraphic or combination traps with respect to sea level cycle phase. Was the reservoir deposited during lowstand, rising, or highstand phases of sea level cycles? What about the seal facies? Is the trap the result of facies relationships that formed during a particular sea level phase or postdepositional deformation?
6. Using information gained in step 5, identify areas that may contain overlooked reservoir, seal, and source rocks in the same isochronous interval or in isochronous intervals with similar character. Also, consider possible migration avenues along which fluids could move from the source rock to the reservoir, from higher to lower pressure regimes. Such avenues might include sand-prone pathways, faults, salt walls, and unconformities.

See also

• Paleogeography
• Constructing a facies map
• Relating traps to paleogeography

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