The Development of Dolomite Geobodies Formed by Geothermal Convection of Seawater: Insights From New Reactive Transport Models Incorporating Platform Growth and Compaction

Abstract

It is estimated that as much as 50% of the world's carbonate rocks are dolomitised (Zenger et al., 1980) and significant volumes of global hydrocarbon resources are contained within dolomite reservoirs. As such, understanding the spatial distribution of dolomitization and how it impacts reservoir quality is key to the characterisation of carbonate reservoirs. Syn-depositional, massive dolomites are generally thought to source Mg from seawater or modified seawater (Land, 1985). Suggested flow systems include circulation within the mixing zone beneath carbonate islands, and reflux of dense brines produced by seawater evaporation in restricted platform top environments. However, both of these scenarios require specific conditions that are only attained intermittently in the lifespan of a carbonate platform. In contrast, geothermal convection of seawater, can occur continuously and is independent of sealevel. Previous reactive transport models (RTMs) of dolomitisation driven by this mechanism have highlighted that it is capable of producing characteristic distributions of dolomite (Whitaker and Xiao, 2012). Dolomite geobodies progressively grow from the platform margin into the platform interior, producing a wedge-shaped dolomite body that thins with distance from the margin. This is calculated to take many millions of years. On this time scale, a carbonate platform can prograde, aggrade or back-step significantly; a factor which is not considered by these simulations. However, the strong spatial association between the platform margin and the final dolomite geobody suggests that an evolving platform geometry could exercise an important control on the final distribution of dolomite within platform successions. We present a series of advanced RTMs that explore this concept. These models consider both platform growth and sediment compaction alongside the geothermal convection mechanism for dolomitisation. The results of these models reveal that the evolution of platform geometry could significatly influence both the amount and distribution of dolomite produced by this mechanism. Specifically, incorporating platform growth within such models does not generate the distinct dolomite geobodies described by Whitaker and Xiao (2012). Instead dolomitisation increases with depth into the carbonate platform as it is within the older, deeper parts of the platform succession that the elevated temperatures required to accelerate rates of dolomitisation exist.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015