Abstract: Constraining Maturation and Yield Timing with Quartz Cement Abundance: Applications in South American Fold and Thrust Belts

David N. Awwiller and Lori L. Summa - Exxon Production Research Co.

Accurate predictions of hydrocarbon trapping potential and oil vs. gas in fold and thrust belts are strongly linked to a good understanding of the relative timing between maturation and thrust emplacement. Therefore, precise thermal and burial history analysis is a key aspect of prospect assessment in fold and thrust belts. The information obtained from conventional paleothermometry techniques (e.g., fluid inclusion, vitrinite reflectance, apatite fission track) is often not sufficient to fully constrain thermal histories. To address this problem, Exxon is currently applying technology that uses quartz cement volumes, predicted from simple precipitation kinetics, to help constrain the time spent near maximum temperature. This information is usually not obtainable from other paleothermometry techniques.

A typical application of the technique is illustrated in Figs. 1-2. Fig. 1 shows two end-member burial scenarios for an Upper Cretaceous source rock in the Llanos foothills: (1) rapid burial in the early Miocene, followed by slower burial until thrust emplacement at ~5 Ma, and (2) slower burial in the early and middle Miocene, with accelerating burial in the late Miocene. Both scenarios are identical after thrust emplacement, indicating uplift at the trap, and continued burial in the trailing edge of the thrust sheet. Neither conventional organic maturity indicators nor apatite fission tracks allow the two scenarios to be differentiated. However, quartz cement abundances in reservoir sandstones preserve additional time/temperature information. Quartz cement volumes calculated using the early burial scenario compare very well with measured values, whereas quartz cement calculated using late burial is much less than the observed value (Fig. 2). We concluded that in this case early burial was most likely, an interpretation supported by subsequent re-evaluation of palynological data. The resulting implications for trap-fill predictions within this thrust sheet are significant. Early burial, which we believe to be most likely, results in a less forgiving scenario for trap fill. More of the source rock has yielded hydrocarbons prior to trap formation, and less of the total yield is available to fill the trap. In addition, there is a higher risk of free gas in the reservoir (Fig. 2).

To date we have used this technique in the Maracaibo basin of western Venezuela, the Llanos and Catatumbo basins of Colombia, and the Santiago basin of Peru. It is particularly appropriate for Late Cretaceous and Paleogene reservoirs of the region, because these reservoirs are typically quartzose, quartz cement is the main pore-filling cement, and they have appropriate thermal histories. In general, the quartz cement paleothermometer is most effective for quartzose sandstones that encompass a broad range of grain sizes and contain little detrital or authigenic clay. The optimum time-temperature window in fold-and-thrust settings ranges from approximately 5 million years at 160-170ºC to 80 million years at 100-110ºC. Outside these bounds, quartz cement paleothermometry may be useful, but some sandstones (e.g., very coarse or very fine grained samples) may be completely cemented or not cemented at all.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil