Print this pageRole of Clay Diagenesis in Hydrocarbon Accumulation in Shale Reservoirs of Sanand Field, Cambay Basin, Gujarat, India: A Case Study
The Early Eocene (Ypresian) Chhatral Member of Cambay Formation in Sanand field has a number of hydrocarbon accumulations in shale.The pay zones are very thin, heterogeneous and developed as discontinuous laminations and lenses. The detection and evaluation of these reservoirs is difficult with electrologs,as resistivity in shale is very low(1-2Ωm) and not much variations seen against hydrocarbon bearing and non hydrocarbon bearing intervals in subsurface.The shaleis grey,fissile having average TOC 3.0% and is immature to early mature with VRo 0.44 to 0.55. The shale has been deposited over a wide spread tidal flat and contains Kaolinite and Chlorite as most abundant clay minerals.
A selective diagenetic transformation of the chlorite in to stable kaolinite is found to be responsible for development of reservoir facies in shale. The floating diagenetic kaolinite pellets/ pseudomorphs form the framework grains and form an unconventional heterogeneous reservoir in shale.Most of the chlorite in shale is transformed in to kaolinite pseudomorphs in an acidic milieu during late stage of diagenesis. With the help of detailed microfacies, petrophysical and Geochemical studies a number of such flow units having better reservoir characteristics were demarcated, and were also recognized in the logs by generating high resolution Scaled Synthetic Resistivity (SRES) curves after processing the Resistivity image data.
The capillary pressure studies indicate a bimodal connected pore sub-system, comprising micropores largely or completely saturated with high capillary bound water and a coexisting system of larger pores which may have high hydrocarbon saturation and form the reservoir units having better flowing characteristics and pore geometry. The degree of transformation of chlorite into more stable kaolinite and textural heterogeneity at meso and micro levels is the main criteria of measuring the reservoir capacity of Chhatral Shale.
Rock Eval parameters indicate high S1 and high PI values which suggests presence of accumulated hydrocarbons in the flow units.Thermal extracts show normal distribution pattern without any signature of biodegradation and the fingerprints match exactly with same genetic attributes. Both the thermogram patterns are different from their respective pyrograms,which indicate that the hydrocarbons in shale are of migratory nature.Small fractures developed in sideritic shale appears to have acted as pathways for migration of hydrocarbon.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil