Understanding Hydrocarbon Accumulations in Ancient Evaporite-Associated Petroleum Systems

Warren, John K.¹
(1)Petroleum Geosciences, Chulalongkorn University, Bangkok, Thailand.

Some, but not all, types of evaporite deposits can be related to likely occurrences of substantial hydrocarbon accumulations hosted by carbonate or siliciclastic reservoirs in suprasalt, intersalt or subsalt positions. These ties and associations can be used to build predictive region-scale exploration models, but only when age-appropriate, tectonic-appropriate and hydrology-appropriate scaling parameters are chosen. Yet, for the past 50 years, inappropriate Quaternary-based sabkha-salina coastal-edge deposit models have used to build interpretations of ancient marine evaporite systems. Many such Quaternary-biased comparisons do not recognise their inherently time-limited and eustatic-limited nature. Nor can they account for scaling and tectonic errors created when using Quaternary analogs in an attempt to create a predictive understanding of hydrocarbon accumulations within regional ancient evaporite paradigms. Predictive understanding is only achieved when the comparison paradigm is built on an appreciation of the greater depositional breadth and diversity inherent to ancient evaporite systems.

Ancient mega-evaporite deposits (platform and/or basinwide deposits) require conditions epeiric seaways (greenhouse climate) and/or continent-continent proximity at the plate-tectonic scale. Basinwide evaporite deposition is facilitated by continent-continent proximity at tectonic plate margins (Late stage E through stage B in the Wilson cycle). This creates an isostatic response where, in an appropriate arid climate belt, large portions of the collision suture belt or the incipient opening rift can be subsealevel, hydrographically isolated (a marine evaporite drawdown basin) and yet fed seawater by a combination of ongoing seepage and occasional marine overflow (Aptian Salt Basins of southern Atlantic vs Miocene Zagros collision belt). Basinwide evaporite deposits can be classified by tectonic setting into: convergent (collision basin), divergent (rift basin; prerift, synrift and postrift) and intracratonic settings. Ancient platform evaporites can be a subset of basinwide deposits, especially in intracratonic sag basins, or part of a widespread epeiric marine platform fill. The latter tend to be mega-sulphates and are associated with hydrographically isolated marine fed saltern and evaporitic mudflat systems in a greenhouse climatic setting (e.g. Ghawar, Saudi Arabia vs. San Andres, West Texas).

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.