Lithofacies Analyses And Diagenetic Evaluation of the Reservoir Units in Well XYZ Field, Onshore, Niger Delta, Nigeria.

ABSTRACT

Lithofacies and petrophysical analysis formed the basis for the diagenetic evaluation of the reservoir units in well XYZ 006, XYZ field, Onshore, Niger Delta, Nigeria. Detailed examination of cores taken from the Lithologic section in XYZ-006 well, led to the recognition of several distinctive component facies comprising of fourteen (14) facies. These was subjected to facies analysis using the Markov Chain statistical technique. The modal analysis of the studied core samples exhibits a comparatively simple mineralogy dominated by quartz occurring as sub-angular to sub-rounded colourless to faint coloured grains grains, making up the bulk of these sandstones. Feldspar and rock fragments were minor. The matrix is made up of kaolinitic clay while cement is dominantly calcite. The individual grains displayed a point to floating point contact. This is consistent with the friable to loosely consolidated nature of the sandstone. Petrophysical evaluation of the cores showed a permeability range of 1- 100millidacies and porosity range of 5-30 %. These parameters was observed to be facies influenced with heterolithic and muddy sandstone having lower porosity. Diagenetic process commenced with mechanical compaction followed by dissolution of shell fragments and reprecipitation as calcite. Additional carbonate cement was precipitated from fluids moving up dip through the sands. Clays present in pore spaces between sand grains altered to more stable forms and insulated some grains from further dissolution or later quartz overgrowths. This was followed by quartz overgrowth formation and finally by hydrocarbons were generated from neighboring source beds. Eight sedimentological distinct Facies association were observed in the cored samples deposited in a continental shelf environment. The Facies comprised of good quality reservoir sandstones inter-bedded with poor quality mudstones. A facies model was generated from the difference matrix where facies with positive outcomes were chosen in lieu of those with negative outcome showing a sedimentary fill typical of shoreface to estuarine environment. The depositional model for the sediments can be summarized as: a prograding lower shallow marine sequence and an overlying upper transgressive estuarine fill