Abstract: Porosity Prediction for Deeply-buried Quartz Arenites in the Llanos Foothills, Eastern Cordillera, Colombia

Warren, E. A.; J. O'Leary; R. Herbert - BP; A.J. Pulham - Univ. of Colorado; R. Graham - Monument; and L. Bonnell - Geologica

Sandstones of the Eocene Mirador formation are the major reservoir target in the Llanos Foothills of Colombia. They are quartz arenites heavily cemented by quartz. Despite low porosity (typically less than 10p.u.) they retain high permeability (up to 5000md) where coarse grained. Porosity is a critical exploration risk because the structures are deep, in excess of 12,000 feet and structural restoration indicates that the sandstones may have been buried over 3000 feet deeper prior to thrusting and uplift. Minus-cement porosity, or intergranular volume, of the sandstones vary from 15-25%, which, when compared to the porosity evolution of a compaction curve (Fig. 1), is consistent with quartz cementation occurring during deep burial. Textural examination of grain and cement contacts has revealed significant variation in the amount of stress compaction (or pressure solution) between different thrust sheets. Most of the stress compaction appears to post-date quartz cementation. Very little pressure solution is observed in the Cusiana thrust sheet, whereas in the Dele thrust sheet, stress compaction features are abundant. Fluid inclusion data from quartz cements indicate that quartz cementation occurred over a broad temperature range between 80 and 150°C. Significant differences were found in the onset temperature of quartz cementation in different thrust sheets. Very low abundances of primary petroleum inclusions in general indicates that most quartz cementation pre-dated petroleum emplacement. The porosity evolution of the Mirador Fmn in the different thrust sheets was unraveled using the petrographic and fluid inclusion data referenced to a compaction curve for rigid-grained sands. The fluid inclusion data establish the depth of quartz cementation using the geothermal gradient of the region. The present porosity of all thrust sheets is much less than that predicted from a compaction curve at this depth, indicating that significant porosity loss has occurred subsequent to compaction. The amount of quartz cement quantified by petrography was used to predict the porosity resulting from compaction and quartz cementation. For the Cusiana thrust sheet, this porosity is the same as the present porosity - indicating that compaction and quartz cementation together account for the porosity loss in this thrust sheet. This was consistent with the porosity predicted using a kinetic model of quartz cementation and the Cusiana burial history. In contrast, the predicted porosity for other thrust sheets was much higher than their present porosity, consistent with additional components of porosity-loss due to stress compaction (up to 10p.u.) and late kaolinite (2-4p.u.). Two models may cause of the stress compaction: deep burial, or thrust-related shortening. Estimates of maximum burial depth for the various thrust sheets in the Llanos Foothills were made based on structural restorations and estimates of lost overburden. These were used to obtain a plot of porosity versus maximum burial depth (Fig. 2). This indicates that any quartz arenite buried in excess of 22000 feet is unlikely to retain porosity higher than 4p.u. Alternatively, the porosity loss attributed to stress compaction is the result of shortening. However, current estimates of shortening in the Llanos Foothills do not reveal an obvious relation between present porosity and the amount of shortening.

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