--> Abstract: Using Basin Modeling as a Predictive Tool in Oil to Gas Cracking Plays, by J. R. Allwardt and G. E. Michael; #90091 (2009)

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Using Basin Modeling as a Predictive Tool in Oil to Gas Cracking Plays

J. R. Allwardt and G. E. Michael
ConocoPhillips, 600 N. Dairy Ashford, Houston, TX, 77079

This study demonstrates the utility of using 2D and 3D basin models for assessing the critical aspects of an oil-to-gas cracking play. The Jurassic Bossier formation of east Texas, USA, represents a good test case for modeling high-temperature, high-pressure petroleum systems. These reservoir sands can be overpressured by as much as 6900 psi (48 MPa, 17.9 ppg) at 14,500 feet (4420 meters) and are sealed by thick sequences of marine to deltaic shales. Current reservoir temperatures are 320 to 370 oF (160 to 188 C). Previous work has proposed that the primary overpressure source in the Bossier is the volume expansion resulting from oil to gas cracking (Chaouche, 2006). Analysis of about 250 wells shows that there is a strong regional correlation between corrected-bottom hole temperatures and overpressure where significant overpressure does not exist below ~300 oF (Figure 1).The three-dimensional basin model, calibrated to about 100 wells, suggests the timing of first oil expulsion ranges from 120 to 70 Ma where the large range is dominantly the result of differences in overburden thickness. Additionally, maps of source rock quality (%TOC) and thickness were approximated from sonic and deep resistivity logs (after Passey et al. 1990) and erosion isopachs were approximated using bulk density curves.

Basin modeling has been used to provide support for the assertion that oil to gas cracking is the dominant overpressure mechanism for the Bossier play. For a closed system, Barker (1990) and Berg and Gangi (1999) have shown that a small percentage of oil cracking to gas can cause significant overpressures that could significantly exceed the lithostatic pressure. This study uses the thermal solution from the 3D basin model to derive the fraction of accumulated oil that has likely cracked to gas and then use the available published equations from Berg and Gangi (1999) to approximate the pressure increase due to secondary cracking. These calculations suggest that the amount of oil to gas cracking in the Bossier has generated large amounts of overpressure and, in some areas, likely reached the fracture gradient up to about 60 myr ago. Also, regions with the highest predicted amounts of cracking correspond to regions of known overpressure. Sedimentation rates are sufficiently low (<2000 ft/myr) that extremely low Bossier shale permeabilities (< 0.1 nD) are required to generate and retain even modest overpressures. This simple modeling exercise further supports that disequilibrium compaction is not the primary pressure mechanism driving the extreme overpressure in the Bossier.

Future work will concentrate on a more robust implementation of the pressure generation from secondary cracking into dynamic flow basin models.

 Figure 1: Temperature- mudweight relationship for 250 wells.

 

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.