Effect of Carbonate Diagenesis on Reservoir Quality of the Middle Bakken Formation, USA

Abstract

The largest continuous oil accumulation ever assessed by the USGS, the Bakken Formation in the Williston Basin, USA, is a mixed conventional and unconventional oil resource which has gained significant interest with recent developments in horizontal drilling technology. Most oil is produced from the middle Bakken unit, a heterogeneous siltstone/sandstone unit in which variable amounts of carbonate cement exert a key control on porosity and thus reservoir quality. In carbonate-cemented zones, characterisation of the pore system by mercury intrusion experiments reveal pore throats <40nm in diameter, comparable to shale. Imaging of pores using highly polished BIB-SEM (broad ion beam) images supports this finding, indicating that carbonate-cemented zones of the middle Bakken are predominantly mesoporous. Whilst porosity has been preserved and created at the core of some dolomite phases, these pores which are unconnected do not contribute to the reservoir quality and are not detected by direct porosity measurements. Whilst clay minerals are present within pores, carbonate cement exerts a first order control on porosity and reservoir quality in the middle Bakken. Nevertheless, whilst XRD analysis indicates that regions of higher porosity correlate with low dolomite abundance, detailed petrographic examination by electron microscopy reveals multiple phases of calcite, dolomite and ferroan dolomite, each of which contributes individually to the degradation of reservoir quality. Previous studies have struggled to unravel the timing and history of porosity-occluding carbonate cement due to its exceptionally fine-grained nature. We have used both sequential acid dissolution and, more importantly, in situ Secondary Ion Mass Spectrometry, to determine the carbon and oxygen isotope composition of individual carbonate phases. The range of carbon and oxygen values is large and suggests multiple phases of cementation, allowing a more nuanced understanding and thus prediction of how carbonate cementation has influenced the destruction, preservation and creation of porosity.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016