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Sediment Process Controls from Clay Mineral Distribution in Modern Estuaries: Predicting Chlorite Cement in Shallow Marine Sandstone Reservoirs

Dowey, Patrick J.1; Byrne, Gemma M.1; Hodgson, David 1; Worden, Richard 1
1 Earth & Ocean Sciences, University of Liverpool, Liverpool, United Kingdom.

Chlorite cement in petroleum reservoirs can be responsible for preservation of anomalously good porosity and permeability in deeply buried sandstones. Fe-chlorite cement is common in reservoirs of estuarine and shallow marine rocks. An understanding of the origin and distribution of chlorite precursor minerals in modern environments could be used as a predictive tool to assess reservoir quality in the subsurface during exploration, appraisal and reservoir development.

Chlorite cement results from burial diagenetic alteration of primary or early diagenetic minerals. Literature suggests that Fe-chlorite forms due to replacement of early diagenetic precursor minerals, such as Fe-rich 7Å berthierine. Present-day estuarine geochemical analyses have indicated that only 5% of dissolved or colloidal iron in river waters reaches open marine environments. Previous work has demonstrated that this Fe-rich clay can occur as a grain coat on sand grains due to animal-sediment interaction. Estuaries are therefore a critical link between fluvial and shallow marine environments, and essential in understanding the origin and distribution of Fe-chlorite minerals in the subsurface.

One of the primary controls on the distribution of Fe-rich precursor minerals is proposed to be shallow marine sedimentary processes, and in particular those intrinsic to estuarine settings. To understand the origin, and to establish a methodology to predict the distribution of these minerals, sedimentological studies have been undertaken in modern estuaries. Sampling methods include estuary mapping and facies descriptions, sediment coring and grab-sampling. A series of analytical procedures have been undertaken including quantitative X-ray diffraction, infrared spectroscopy, various scanning electron microscopy techniques and optical cathodeluminescence microscopy to establish precursor mineral type, structure, distribution, and relative abundance. Particle size analysis was also used to establish sediment grain size distribution at each location. Initial analyses indicate preferential sites of Fe-chlorite precursor mineral deposition under the influence of dominant sediment processes.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009