Estimating Brittleness Index Using Mineralogy and TOC in the Whangai Formation, New Zealand

Abstract

Abstract

Currently, shallow and light-oil reservoirs are can be difficult to find, and operators are exploring and developing complex, risky, and therefore costly reservoirs. These factors are turning operators to look for opportunities in unconventional resources, like shale oil and shale gas around the world. The Whangai Formation, located in the Southern East Coast Basin of New Zealand, has been identified as a good potential source rock to become an oil producer. The reservoir properties of Whangai Formation are similar to the Bakken Shale in the U.S., which is currently one of the two most-active shale plays.

In this study, three offset wells with conventional log data, mudlog data and total organic carbon (TOC) laboratory tests were analyzed to evaluate the mineralogical composition and to calculate the TOC content profile. Based on this, we have integrated studies of cuttings analysis (TOC and Rock-Eval pyrolysis) and well logs (GR, NEU, DEN, DT, RES) for the mineralogical characterization. The Passey equation was used for TOC prediction, to compare with the cutting analysis and create a TOC log profile, and petrophysics analysis for the mineralogical characterization. The brittleness index (BI) using Javier et al. (2007) and Wang and Gale (2009) correlations were used to estimate BI and to identify sweet spot zones.

The results indicate that the Whangai Formation is a non-calcareous siliceous mudstone comprising approximately 20% sands, 15% carbonates, and 70% clays, with a TOC of 2%. The results are indicating a discrepancy between both results, once BI using Javier's equation is about 15% and using Wang is around 20% and the best brittleness zone is on the middle-to-top zone. Additionally, a mineralogical lateral variation was identified, affecting the brittleness evaluation. In this article we describe the applicability of both methodologies and discuss their accuracy.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016