AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Chlorite Cement in Sandstone Reservoirs: Analogue Studies of Clay Formation Processes in Modern Estuaries
(1) Earth & Ocean Sciences, University of Liverpool, Liverpool, United Kingdom.
The presence of iron-rich chlorite cements can inhibit quartz cement growth, leading to enhanced porosity and permeability in deeply buried petroleum reservoirs. Knowledge of the origin and distribution of iron-rich chlorite will be utilised to predict good reservoir quality in the subsurface. Iron-rich chlorite is not a primary mineral; it forms due to the replacement of early diagenetic precursor minerals. The focus of this analogue study is to understand the chemical constraints that control growth of iron-rich clays which, in turn, are diagenetically altered to form iron-rich chlorite.
Iron-rich chlorite is associated with estuarine and deltaic sedimentary rocks; consequently this analogue study is focused on chemical changes within a modern estuary. Supply of iron into the sedimentary system is required for the formation of iron-rich clays; previous estuary water geochemical studies have shown that 95% of transported iron in rivers is deposited in estuaries. Thus a focus was placed on hinterland geology and the local weathering regime as primary controls on the origin of iron-rich precursor clays.
The Leirárvogur estuary, south-west Iceland, was chosen as an analogue study. The basaltic hinterland geology presents a rich supply of iron into the fluvial system, which in turn may be utilised to form iron-rich clays in the estuary. To understand the water geochemical constraints on iron-rich clay mineral precipitation, a full suite of water chemistries is required. It is important to understand the relationship between the changing chemistry where and when fresh and marine waters mix and the suspended particulate matter within the water column at any one point in time throughout a tidal cycle.
At static points around the estuary over tidal cycles, water samples were collected, filtered and preserved. Fresh and marine samples were also collected. Field analysis and ion chromatography have been used to determine cation, anion and iron concentrations. Clay mineral quantification and identification of the suspended particulate matter has been carried out using XRD, IR and SEM. Initial results show Fe-rich clay and semi amorphous Fe hydroxide (created in soils) are transported in rivers, along with truly dissolved iron. These are deposited where river water meets seawater in the estuary; this site varies in time and space during the tide cycle. Results are beginning to reveal where chlorite-precursor minerals are first deposited in modern estuaries.