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Chapter 6 Evaporative Marine Diagenetic Environments

Publisher
Elsevier Science & Technology
Identifiers
DOI: 10.1016/s0070-4571(01)80007-8
Disciplines
  • Chemistry
  • Earth Science
  • Geography

Abstract

Summary Some of the most important oil and gas fields around the world are developed in dolomite reservoirs related to diagenesis in evaporative marine environments. Evaporites seal reservoirs and occasionally destroy porosity by cementation. The precipitation of gypsum contributes significantly to dolomitization via incorporation of calcium ions into the gypsum crystals and the resultant dramatic increase in the Mg/Ca in associated fluids. Effective porosity development in sabkha-related dolomites often depends on post-dolomitization fresh water flushing as a consequence of exposure and formation of sequence boundary diagenetic terrains. Marginal marine evaporative sequences occur in two major settings: the sabkha, where evaporites and dolomites form under dominantly subaerial conditions; and the barred evaporite lagoon or basin, where evaporates are formed subaqueously. Sabkha dolomitization is limited by a restricted hydrologic system, although adjacent porous limestones often tend to be dolomitized by refluxed brines. There are two basic settings for the barred evaporite basin or lagoon. The first is where the barrier that restricts marine water influxing into a shelf or basin does not totally isolate the basin or lagoon from the open sea. In this case, lateral and vertical density gradients are established in the lagoon. Continued influx of marine waters allows the deposition of gypsum in the landward reaches of the lagoon and reflux of post-gypsum Mg-rich brines into the previously deposited sequence. Under these conditions both extensive reflux dolomitization of the underlying sequence and porosity enhancement are possible. The lagoonal evaporite reflux setting is commonly developed during the transgressive systems tract, where significant volumes of new accommodation space form and barring of the shelf/basin is incomplete. The second setting is the coastal salina where the salina or lagoon is totally cut off from marine water influx by the barrier. Under these conditions evaporative drawdown is operative, and evaporites and fine-grained carbonates are deposited across the entire lagoon, effectively sealing the sequences below from brine reflux and subsequent dolomitization. Halite and even bittern salts are commonly precipitated along with gypsum and anhydrite. The coastal salina setting is commonly developed during periods of slow accommodation space formation, such as during the late highstand systems tract (HST) or the lowstand systems tract (LST). Dolomitization and porosity enhancement are less likely in such a setting, although the associated evaporites can provide an excellent seal for any pre-existing porous carbonates. Geologic setting, timing of dolomitization, relationship of dolomite and evaporite sequences, as well as traces of leached evaporites, such as collapse breccias, are fundamental to the recognition of evaporative marine diagenesis. Although the chemical and isotopic signature of modern evaporative marine dolomites is distinctive, great care must be exercised in using dolomite geochemistry because of the possibility of metastable dolomite recrystallization.

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