Stratigraphic High-Grading of Exploration Fairways: Deepwater Reservoir Distribution in Time and Space, Offshore East Africa

Abstract

Abstract

The area offshore Tanzania and Mozambique continues to emerge as a significant global gas province, with estimated recoverable reserves of 100 TCF from recent discoveries in Cretaceous and Early Tertiary deepwater fan reservoirs. As opportunities move into deeper water offshore, the challenge becomes high-grading the many exploration blocks in this vast region.

Fan/channel systems in the established offshore Rovuma Basin exhibit remarkable reservoir properties and high connectivity; however, predicting their spatial/temporal distribution remains a critical exploration problem. We reconstruct regional chronostratigraphy by generating high-resolution stratal packages from 2D long-record, long-offset SPAN surveys. Initially we generate a HorizonCube (closely-vertically-spaced horizon interpretation) on each 2D survey. Then each of these horizon sets is flattened to create a chronostratigraphic (“Wheeler”) representation of sedimentation along that survey. We delineate stratal packages by comparing and correlating multiple such geochronological representations of the seismic data. We then analyze stratigraphic architecture within packages, by evaluating internal geometries as well as utilizing a horizon density attribute to assess sedimentation rates. Our methodology reveals systematic, chronostratigraphic separation of reservoir geometries and architectural dimensions.

Architectural elements and their dimensions vary according to the depositional gradient, but also to stratigraphic level. Steeper geomorphic profiles generate more up-dip incision along with higher sediment accumulation rates down dip. Channelization is concentrated, resulting in a stratigraphic record composed of multi-story channel systems, fewer levees and smaller, multi-storied fan lobes. Gentler depositional gradients have lower sediment accumulation rates, yielding single-storied channels, symmetric levees and larger, shingled lobes and splays. Channel/lobe dimensions and aspect ratios vary with steepness of the depositional profile.

Our finding that mapped reservoir architectural elements vary systematically through time and space has significant implications for predicting connectivity and reservoir distribution. By providing fairway delineation and reservoir property prediction from a coarse grid of regional 2D data, application of this work can focus and de-risk exploration efforts.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016