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Geochemical Constraints Affecting the Growth of Chlorite Cement in Petroleum Reservoirs, Using Modern Estuaries as Analogues for the Subsurface

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

Porosity and permeability in deeply buried petroleum reservoirs can be radically enhanced due to the presence of iron-rich chlorite cements since chlorite grain coats seem to inhibit quartz cement growth. Understanding the origin and distribution of iron-rich chlorite is a long-term goal to help predict anomalously good reservoir quality in the subsurface. Iron-rich chlorite is not a primary mineral but forms as a result of burial diagenetic changes to detrital iron-rich clays.

Iron-rich chlorite is usually associated with estuarine and deltaic sedimentary rocks. In fully marine sediments, it is widely held that sulphate reduction and consequent growth of sulphide minerals means that iron-rich clay minerals seldom develop. Siderite tends to be the dominant iron-mineral that develops in humid continental sediments. Iron-minerals are of limited importance in arid continental sediments. Iron-rich chlorite therefore seems to be restricted to environments intermediate between continental and marine.

Having a supply of iron into the original sedimentary system must be the primary control on whether iron-rich chlorite occurs within sediment. Hinterland geology and weathering regime are important factors. However, river water geochemical studies have shown that 95% of transported iron in rivers is deposited in estuaries; only a small quantity makes it out into the open sea. This suggests that estuaries are traps for iron and thus iron-rich minerals. Flocculation and microbiological sediment interaction processes are all likely to be involved in this trapping process. Previous studies at the University of Liverpool have also shown that iron-rich clays can form as grain coats on sand grains due to animal-sediment interactions meaning that environment-specific animal populations may also have a major effect on localisation on growth of these clay minerals.

To consider and quantify this array of potential controls and influences, and to establish a method by which iron-chlorite distribution can be predicted in estuarine and deltaic settings, mineralogical and geochemical analyses are being undertaken in modern estuaries. Sampling methods include estuary mapping, sediment coring, water sampling, salinity, and temperature and pH changes. Analytical procedures will include the use of quantitative X-ray diffraction and FT-IR spectroscopy, ion chromatography and voltammetric analysis.


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