Modelling Stratigraphic Controls on Dolomitisation at the Inter-well Scale: The Assoul Formation Case Study (Central High Atlas, Morocco)

Abstract

Conceptual models predicting dolomite distribution and its effect on reservoir quality are defined by the hydrological system driving the fluid flow and by the fluid chemistry, and suggest systematic and gradual changes in dolomite abundance controlled by the geometry of the flow system. Such simple models ignore both dependence on depositional texture and feedback during diagenesis. These factors can all but obscure any systematic trends in the distribution of diagenetic products. Preferential dolomitisation of mudstone beds is common in the rock record, but there are also systems in which dolomitisation favours grainstone units rather than mudstones. The prediction of diagenetic reservoir quality thus depends on the identification and understanding of the parameters that control this behaviour. In this study we perform a series of reactive transport model (RTM) simulations using a 2D slice of an outcrop-based facies model of a Jurassic carbonate ramp from Morocco. High resolution simulations (cell size 15m × average 0.3m) of fluid flow and reactions at the inter-well scale (750m × 120m) were developed from the geological model in order to explore how spatial variations in sediment texture influence fluid flow and dolomitisation by seawater-derived brines at 40-100°C. This high resolution approach allows us to assess the control on dolomitisation imposed by depositional heterogeneities at scales ranging from the stacking patterns of depositional sequences, to facies associations and down to the individual facies types. Simulations of bedding-parallel flow through a sequence with permeability contrast between grain- and mud-supported facies ranging over 4-5 orders of magnitude show preferential dolomitisation of the more permeable grainstones despite the higher reactivity of the mudstones. Reducing the permeability contrast by increasing mudstone permeability produces a more complex pattern of dolomitisation, reflecting a combined effect of fluid flow and reactivity. Sharp reaction fronts tend to develop at higher temperature when textural variations are present as reactions serve to enhance permeability and/or reactivity. In contrast, when fluid flow is perpendicular to the bedding, diagenetic alteration tends to be more pervasive and to affect first the fine-grained, more reactive sediments. RTMs can thus help explore thresholds in the operation of diagenetic systems, and seek to identify emerging behaviours which can influence the reservoir quality.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017