Solutions to Problems Faced in Well Log Analysis in Carbonate Reservoir Systems Due To Multiple Porosities and Lithologies

Sharma, Akash; Jain, Tarang; Kapoor, Harsh
Department of Petroleum Engineering and Earth Sciences, University of Petroleum and Energy Studies, Dehradun, India.

Carbonate rocks are the most abundant non-terrigenous sedimentary rocks and are composed primarily of carbonate minerals. Most carbonate rocks form from calcareous deposits that accumulate in marine environments ranging from tidal flats to reefs to deep ocean basins. Distinctive and unique aspects of carbonate rocks are their predominantly intrabasinal origin, their primary dependence on organic activities for their constituents and their susceptibility to modification by post-depositional mechanisms.

In conventional analysis of Wireline logs of carbonates, the results are commonly not reliable. There are primarily two reasons. Firstly, the rocks are composed of two intermingled carbonate litholigies, namely dolomite and calcite. Secondly, Porosity distributions commonly encompass a wide range of values.

In recent developments, such problems were encountered in the Campos Basin, offshore Brazil. Intensive drilling and seismic, gravity and magnetic data have contributed to the recognition of four tectono-stratigraphic units related to the rifting and break-up of Pangaea. Its most important Sag/Macabu Formation has been characterized as microbial and stromatolitic limestones, locally dolomitized and often silicified. Log correlation is very challenging in the Macabu Formationc due to large scale heterogeneities and variable porosity characteristics within the Sag section, which is divided into three different porosity units: Upper, Middle and Lower Sag. The pay interval to date has been limited to the Upper Sag in the Campos Basin wells. However, the Lower Sag is productive and has significant reservoir potential. Here, companies are using a combination various technologies and methods for proper interpretation and understanding of logging data.

The present paper reviews the various challenges of log analysis in carbonate reservoirs and also proposes solutions such as comparison of Density and Neutron logs as its combination provides a robust method of differentiating dolomite from limestone. For porosity related issues, integrating log analysis with core studies provides a handy solution as it quantifies macropores and understands connections and permeability. NMR measurements reveal pore and fluid properties in rock formations through a two-stage measurement. Various approaches have been suggested for this ranging from empirical methods to theoretical methods.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012