Petrophysical Characterization of Mineralogically Complex Panna Formation: An Alternate Approach

Ghosh, Arnab (Schlumberger) | Zacharia, Joseph (Schlumberger) | Majithia, Pritpal Singh (ONGC Ltd.) | Saxena, Renu (ONGC Ltd.) | Nirala, Arun Kumar (ONGC Ltd.) | Narla, Sattibabu (ONGC Ltd.)

OnePetro 

Abstract

Panna Formation is a very critical and challenging formation deposited during Paleocene time of geological past in various parts of Western Offshore Basin of India. It was deposited in a fluvio-deltaic environment, sometimes even in a restricted marine set-up. Such variation in depositional setting caused mineralogical complexity, which in-turn imposed a limitation in conventional approach of formation evaluation and saturation determination to identify the pay zones with confidence. A comprehensive approach of integrated formation evaluation for rock quality characterization was attempted using a combination of new generation elemental and acoustic analysis for delineating the potential hydrocarbon bearing zones independent of conventional resistivity-based approach along with a better insight on formation heterogeneity. The studied well was drilled up to Panna Formation and conventional open-hole logs were acquired while drilling. However, due to complex mineralogical nature of the formation, estimation of key critical reservoir parameters was very challenging and imposed higher uncertainties in the results. An alternate approach was adopted using a few advanced log measurements to address this challenge. A combination of new generation elemental and acoustic data has been recorded in a single wireline run after acquiring conventional basic logs while drilling. An accurate porosity was derived by eliminating various mineralogical assemblages along with estimation of a geochemical permeability based on detailed elemental analysis. Measured aluminum from neutron inelastic capture spectrum method enabled to estimate clay volumes with accuracy, which provided the required insight for better effective porosity in such shaly-sand environment. Based on this improved porosity and permeability, an approach for rock-quality indexing was used for reservoir delineation.

Moreover, a good amount of organic carbon was found associated with clays caused shales with higher resistivity. Based on elemental measurements an interesting insight was possible to extract for resistivity independent fluid saturation. An additional pay zone with hydrocarbon saturation based on such resistivity independent approach was possible to identify, which was masked by conventional resistivity-based interpretation. Acoustic analysis results assisted in delineating the zones with possible open fractures to avoid any possibility for unwanted fluid breakthrough.

Based on this approach of alternate integrated petrophysical analysis perforation zones were selected including an additional zone, which was masked based on conventional analysis. The well was started producing around 1,05,000 m3 gas with around 200 barrels of oil per day. This study showcased an alternate and efficient characterization approach for any such mineralogically challenging clastic formations.