Abstract/Excerpt

Constraining Palaeo Heat Flow for Petroleum Systems Modeling Using a Combined FlexDecomp, Stretch, Gravity Inversion and SfMargin Modeling Workflow

Jonathan McQuilken, David Dickson, and Neil Hurst
BG Group, Thames Valley Park, Reading, Berkshire, UK

When undertaking a Petroleum Systems Modeling (PSM) project there are many unknowns. One unknown, which has a major impact on the outcome, is palaeo heat flow. Present day heat flow is often the starting point for constraining palaeo heat flow and is easy to define from the literature or calculated from BHTs. Once defined it is extrapolated through time using knowledge of palaeo heat flow profiles such as those for a rift basin, but this is only a good estimate.

At the initial 2D petroleum systems project framing for the Qiongdongnan Basin, South China Sea, it was clear that the published heat flow(1) could be refined and built upon to allow palaeo heat flow for the modeled line to be more robustly constrained using a combined FlexDecomp, Stretch, Gravity Inversion and SfMargin Modeling workflow and was seen as best practice for understanding basin development history. This workflow starts with the determination of crustal thinning from seismic and gravity modelling. Independently, stretching estimates are calculated by restoring palaeobathymetric markers to sea level or other control points using FlexDecomp which uses a uniform lithospheric stretching model.(2) If there is a discrepancy between the modeled extension and the observed crustal thinning from the seismic and gravity data, this indicates another additional mechanism of subsidence is at play. SfMargin forward models the basin to match the subsidence and thinning patterns using more complicated models including depth dependent stretching, multiple rift events, underplating and volcanism. Palaeo heat flow profiles are obtained from the stretching factors if the SfMargin model matches the present day basin geometry and honours the regional tectonic evolution.

Prior to 2D PSM, palaeo heat flow profiles were taken from the SfMargin model at three distinct structural domains along the line, the Northern Flank, the Basin Centre and the Southern Flank and differ to the published heat flow. Using these palaeo heat flow profiles has impacted the PSM outcome when compared to using the published heat flow. Overall, the petroleum system is much more gas prone due to the maturity being much greater in the Basin Centre and slightly more mature on the Southern Flanks. This greater maturity has noticeable effects on the timing of generation particularly in the Basin Centre and is earlier by 15 ma.

References

H. Chen, S. Li, Y. Sun and Q. Zhang: Two Petroleum Systems charge the YA13-1 Gas Field in Yinggehai and Qiongdongnan Basins, South China Sea. AAPG Bulletin, V 82, No 5a (May 1998 Part A), pp 757-772.
McKenzie, D., 1978. Some remarks on the development of sedimentary basins, Earth and Planet Sci. lett., v. 40, 25-32.

The combined FlexDecomp, Stretch, Gravity Inversion and SfMargin Modeling workflow was undertaken by Dr. A. Roberts (Badley Geoscience Ltd) and Prof. N. Kusznir, Department of Earth & Ocean Sciences, University of Liverpool.

AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012