--> --> Sediment Budgets From Mass Balance Grain Size Trends, Wasia Formation, Saudi Arabia

2018 AAPG International Conference and Exhibition

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Sediment Budgets From Mass Balance Grain Size Trends, Wasia Formation, Saudi Arabia

Abstract

Volumetric calculations are increasingly important in modern and ancient routing system analysis and basin modelling. They allow prediction of catchment sizes, provenance of sediments, and tectonic uplift rates of continental hinterlands. Provenance, climate, and uplift rates are important parameters that control clay mineralogy, clastic sediment maturity and, therefore, reservoir quality. A common issue is that available subsurface data (well, seismic) of ancient sedimentary systems and hydrocarbon provinces do not cover the whole source-to-sink area. Therefore, calculated sediment volumes underestimate the total volumes released from catchment-source areas and deposited along the sedimentary fairway. This introduces uncertainties in estimated rates of denudation, tectonic uplift, and predictions of catchment sizes. Mass balance grain size trends (MBGT) for sedimentary systems allows geologists to specify the relative amount of sediment volumes preserved within the area of interest versus total sediment volumes released from continental hinterlands and distributed across the sedimentary basin. MBGTs accurately estimate the total volume, sediment flux, and denudation rates for complete sedimentary systems. Mass balance (MB) represents nondimensional, normalized volumetric distance along reconstructed sedimentary pathways. The transformation of grain size trends from distance to MB depends on the shape and volume of the basin. Due to their nondimensional nature, MBGTs from different systems can be compared against “global” MBGTs. This comparison is the basis of the volumetric corrections between measured and total volumes. This study uses an example from the Wasia Formation in eastern onshore and offshore Saudi Arabia to highlight the methodology. The Wasia Formation is an Albian to early Turonian mixed sedimentary system with multiple delta systems in time and space. More than 1000 wells provide abundant datasets with thickness, lithology, grain size, and depositional environment. The depositional system covers approximately 1000 km2 and is sufficiently large to reconstruct large-scale sedimentary pathways and to compare regional with “global” MBGTs. The methodology presented for volumetric corrections and provenance analysis can be applied to any continental-marine sedimentary system, whether preserved at the surface or subsurface. Analysis of the Wasia Formation shows that previous approaches may have underestimated sediment flux by a factor of up to 10x.