Accounting for the Effects of Lateral Stress in Basin and Reservoir Quality Modeling in Compressive Tectonic Environments
Rajmon, David; Hathon, Lori
Accurate prediction of sediment porosity is important in multiple E&P applications including thermal history, reservoir quality, and pore pressure prediction. In quartzose sandstones with low thermal maturities, porosity evolution during burial is controlled primarily by effective stress (compaction). Many workflows assume that the effective stress may be approximated by its vertical component - the vertical effective stress (VES). This simplification is not appropriate in compressive (SH > Sh ≥ SV) or strike-slip (SH > SV ≥ Sh) regimes where horizontal stress components are equal to, or exceed, the vertical stress.
Despite abundant published field evidence, many explorationists continue to underestimate sediment compaction in compressive regimes where the onset of elevated horizontal stresses has occurred prior to significant cement precipitation.
In order to illustrate the problem, we:
1. estimated the magnitudes of the principal stress components using Anderson's faulting theory,
2. transformed them into a single parameter - "equivalent mean effective stress (p*)"- using the Modified Cam Clay model. This parameter accounts for all three principal stress components driving compaction.
3. applied p* in place of VES in the basin and reservoir modeling workflow. Current basin modeling tools do not describe lateral compaction and deformation explicitly. Therefore, we increased lithology density, which led to an increased calculated lithostatic stress and elevated VES (~p*). Simulating increased stress in a basin model allows for capturing effects on thermal conductivity, radioactive heat generation, temperature, permeability and pore pressure.
4. calculated multiple porosity predictions in Touchstone™ for sand samples from NW Borneo assuming realistic compaction parameters and various modified VES histories.
This workflow is a workaround which does not replace a more rigorous stress analysis; however, it does provide practical guidelines for exploration geologists and basin modelers for estimating sediment porosities. As a rough rule of thumb, rocks near reverse faults experience the highest lateral stress and the porosities in this regime are ~8 % (relative to bulk rock) lower than in normally compacted sediments. Increasing VES 2-3 times in existing modeling tools yields approximately correct porosities in this environment.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013