Integrated Cores, Well Logs, MRI and NMR Data for Carbonate Pore Type Characterization: Case Study of the Cretaceous Mishrif Formation in Southeast Iraq

Abstract

To characterize pore type distribution of carbonate rocks is a meaningful work for evaluation of reservoir quality and flow properties. The thin sections and core data with well logs provide ways to link both pore types and rock fabrics to petrophysical responses quantitatively. Take the Cretaceous Mishrif Formation as an example, six core wells from a prolific hydrocarbon reserve located in southeast Iraq have been used to classify carbonate pore types. The Mishrif carbonate rock includes interparticle pores with connected vugs, intraparticle pores with isolated vugs and moldic pores, and micropores. Fractures are rare among multi-scale recordings. Based on conventional well logs, micro-resistivity images and NMR data, detailed characteristics are: (1) interparticle pores and connected vugs correspond to increasing acoustic transit time (AC) and positive separation between deep and medium electrical resistivity measurements. The pores develop in bioclastic limestone within biostrome and shoal facies after undergoing two major meteoric dissolution in lower part of Mishrif intervals. The connected pore system displays aggregated dark color and scattered dark dots under grain-supported light backgrounds in borehole dynamic images. Connected and enlarged pores cause large transverse-relaxation-time (T₂) with right-towards unimodal peaks in NMR spectra. (2) isolated vugs, intraparticle and biomouldic pores are common in packstone and wackstone in back-shoal and lagoon facies or in grainstones experienced marine or early burial cementation. These pores contribute to reducing of AC and moderate positive or weak negative separation between resistivity logs. Nodular grains with calcite cements display light patches in borehole images, along with spotty or separated dark images representing isolated or weak connected pores. This type of pore system causes dominant right peaks with left-hands sub-peaks in T₂ spectra. (3) micropores originate from interparticle pores between micrites, and remained space within calcite or dolomite crystals in lagoon and back-shoal facies, which make AC shorter with lower rock density and weak resistivity differences. Micropore system shows low amplitude left-hands peaks with or without right-hands peaks in T₂ spectra. This study is basic work for spatially geophysical pore system characterization as well as rock-physics models for reservoir evaluation, and also have implication for geologic understanding of carbonate pore system.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018