--> Investigation of in-situ temperature and processes of diagenetic transition from opal-A to opal-CT and their physical/chemical controls in the subsurface San Joaquin Basin, California

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Investigation of in-situ temperature and processes of diagenetic transition from opal-A to opal-CT and their physical/chemical controls in the subsurface San Joaquin Basin, California

Abstract

Controls on diagenetic changes in siliceous mudstones – from opal-A to opal-CT and from opal-CT to quartz silica phases – are not extensively understood and previous studies found large and overlapping temperature windows for these transitions. Constrained and accurate temperature profiles will aid in predicting the depth and timing of diagenesis, determining maximum temperature and burial depth of uplifted strata, and characterizing paragenetic relationships. Wireline logs, whole core, sidewall sample and cutting analysis will be utilized to identify and correlate diagenetic transition zones in the Monterey Formation, San Joaquin Basin, California. Accounting for the paleo- and present-day geothermal gradient is crucial to understanding the original subsurface temperatures to which these rocks were subject. SEM and XRD analysis will be carried out to identify opal-A, opal-A’ (if present) and opal-CT, d-spacing, and related authigenic minerals or diagenetic processes. Recent research also suggests that the opal-A to opal-CT transition is not always simple dissolution and precipitation of the two silica phases and there may be additional processes that result in an intermediate stage and other associated mineralizations. Temperatures at diagenetic transition zones derived from different field areas with distinct thermal gradients or heat flow histories will narrow the individual temperature windows of the opal-A to opal-CT (and possibly the opal-CT to quartz phase) transition and provide information on secondary controls of the transition rate. We hypothesize that the depth and thickness of transition zones at each location will not only relate to the current geothermal gradient but will also relate to local burial and uplift history. This study will address the great spread in the temperatures of the silica phase transitions that have been reported by previous studies, utilizing a variety of different methods. If the depth and temperature of diagenetic transition windows can be better predicted, it may result in changes to current oil well completions and help reduce risk and uncertainty. If opal-A’ is found to be present in certain zones or under certain conditions and is more abundant or widespread than previously thought, it may produce dramatic changes in reservoir characteristics, such as porosity, permeability and reservoir connectivity, quite different than in pristine opal-A diatomites.