--> Quantifying Depositional Architecture of Basin-Floor Fan Pinchouts: Implications for Improved Stratigraphic Trap Prediction

AAPG ACE 2018

Datapages, Inc.Print this page

Quantifying Depositional Architecture of Basin-Floor Fan Pinchouts: Implications for Improved Stratigraphic Trap Prediction

Abstract

Stratigraphic traps occur due to changes in sediment character at the pinchout of sandbodies, and are increasingly identified as economically-viable prospects for exploration in mature and frontier hydrocarbon basins. Stratigraphic pinchout configurations in deep-water fan systems vary due to factors such as the scale and orientation of seabed topography, and flow rheology. Detailed sub-seismic scale analysis of the sedimentology and depositional architecture of lobe pinchouts has been widely documented. However, quantification of the rates of change of these parameters spatially in different palaeogeographic positions and basin configurations is less well addressed.

In this study, we utilize data from oblique up-dip and lateral pinchouts of Fans 3 and 4 in the Tanqua depocentre, Karoo Basin, South Africa, to constrain and quantify the rates of change in gross interval thickness, sedimentary facies and percentage sandstone. Walking out continuous exposures between multiple logged sections, complemented by fully cored research boreholes, permitted detailed characterization along transects from axis to fringe of Fans 3 and 4.

At the oblique up-dip pinchouts the fans thin abruptly at a rate of 12 m/km (Fan 3) and 7 m/km (Fan 4) above a slope with an average gradient of 0.6°. Facies change abruptly from ripple and climbing ripple dominated sandstone units to thin-bedded heterolithics over a distance of 5 km, with percentage sandstone decreasing exponentially for both fans. This pinchout geometry is not a simple sheet-like termination, rather there is localized thickening and reappearance of structured sandstone along the pinchout margin, suggesting a complicated seabed topography. In contrast, the thinning rate of the lateral pinchout is 1.3 m/km for both Fan 3 and 4, downlapping onto a slope with a gradient of 0.1°. Facies change from thick-bedded structureless sandstone to thin-bedded heterolithics over a distance of 12 km. Percentage sandstone decreases linearly with a large number of argillaceous hybrid beds increasing the heterogeneity of the Fan 3 pinchout, whilst they are rare in the lateral pinch out of Fan 4.

This study quantifies differences in the rates of change in fan thickness, facies, and reservoir properties between the oblique up-dip and lateral fan pinchouts at sub-seismic resolutions. These properties are crucial when defining reservoir input parameters for stratigraphic traps in submarine fan systems.