Characterization of Diagenetic Phase Transformations in Lacustrine Biogenic Silica Rich Rocks Using Porosity-Depth Trends in Barmer Basin, India.
Authors: Samal Saphalata , Pal Arindam , Guha Rupdip 
Published: April 2017

Lacustrine sediments of the Barmer Hill Formation are regionally spread throughout the northern part of the Barmer Basin, India. Laminated porcellanites of this formation are hydrocarbon reservoirs of commercial interest. The siliceous sediments were originally deposited as diatom frustules that underwent diagenetic transformation to Opal-A (diatomites) then to metastable Opal-CT (porcellanites) that finally stabilized as microcrystalline quartz. The mineral phase transitions are best characterized by trends in the rock physical properties.

Porcellanites are high porosity (~25p.u.), low permeability (~0.2mD) and moderate strength reservoir rocks. The diagenetic change from unconsolidated Opal-A silica to microcrystalline quartz occurs through a series of dissolution and re-precipitation processes impacting the petrophysical and mechanical properties of the rocks. Partially crystallized porcellanites have been regionally correlated in more than 500 wells within the basin. Porosity trends for these reservoir facies were analyzed in several shale content bins (Vshale windows), across the entire depth range. Necessary depth corrections were applied to offset the effect of basin uplift before comparing the compositional changes and trends of porcellanites across different fields.

Weathered porcellanites in Opal-CT form are seen in surface exposures. In the subsurface, multiple porosity-depth trends are observed in porcellanites that have undergone diagenesis and compaction, simultaneously. A prominent diagenetic trend (6p.u./100m) is observed at shallower depths indicating phase transition from Opal-CT to micro-crystalline quartz. The phase transformation reactions are catalyzed by temperature and detrital content of the rocks. At a greater depth, where most of the sediments are already converted to microcrystalline quartz, the compaction trend (2p.u./100m) dominates. This trend is comparable to the regional shale compaction trend. The porosity trends, especially in Opal-CT, are further complicated by the imprint of overburden compaction and the differential uplift of the basin. The transformation boundaries are not sharp and characterized by a transition zone where porcellanite porosities reduce from about~40p.u. to ~20p.u. The X-ray diffraction data indicate that the microcrystalline quartz grains tend to improve crystallinity within these transition zones. Diagenetic maturity increases from north to south in the Barmer basin with surface exposure of Opal-CT in the northern part and pure microcrystalline quartz in the deeper southern part of the basin.

Porosity-depth trend analysis gives a holistic overview of diagenetic phase transformation in the porcellanite reservoirs of the Barmer Hill formation. Electron Microscopy (SEM) and X-ray diffraction (XRD) data validate mineral phases at various depths in different parts of the basin. For an optimized field development planning of these reservoirs, porcellanites are best characterized by the property trends due to mineral phase transformations.