Powerful Methodology to Un-Mask Sedimentological Characteristics of Carbonates That Heavily Affected by Diagenesis to Map Depositional Trends for Building 3D Static Model
Authors: Aidarbayev Sanat , Qiu Maojun 
Published: October 2023

Abstract

Objectives/Scope
While building 3D static model as part of development activities of hydrocarbon reservoirs, characterizing and incorporating geological understanding remain as one of the key challenges. The geological understanding of carbonates is broadly composed of three stages: understanding of depositional facies, followed by their diagenetic alterations, and later affected by deformations (fractures, compaction, etc.). These stages should be understood step-by-step following geological evolution of a reservoir. However, geologists who describe cores and interpret depositional environments often find it difficult to correlate to available logs. Majority if not all logs record subsurface rock characteristics that went through many alterations and represent final product, where initial depositional characteristics are masked or lost. This paper presents a simple but powerful methodology to un-mask those properties imprinted during sedimentation and to build meaningful depositional maps.

Methodology
Standard set of triple combo logs are normally acquired from all drilled wells. These logs are bulk density (RHOB), neutron porosity (NPHI), and resistivity (RS, RD, RXO), plus sonic (DT) as auxiliary log. RHOB and NPHI are affected by mainly diagenesis and compaction, resistivity greatly varies by type of fluids in pore space, while DT log can provide an indication of primary porosity, but it is still sensitive to fractures. Therefore, despite of their abundance, none of them can help to fully retrieve initial sedimentological characteristics. Fortunately, another important, abundant and under-estimated log is Gamma Ray (GR). Commonly, GR is used only for picking markers, isolating reservoir from non-reservoir, and/or calculating a volume of shale. GR records levels of Uranium, Thorium and Potassium radioactive elements which are considered as stable and resistant to post-depositional reactions. Consequently, it makes GR an irreplaceable tool for carbonates where initial depositional facies were altered by diagenesis. To make sure that logs are consistent throughout all wells, standard log processing QC steps have been applied such as depth shifting, and normalization.

Results
Based on 60 analyzed wells; R1 reservoir composes of depositionally best shoal facies and near-shoal facies. Both facies are heavily cemented in R1 Upper and no cementation in R1 Lower. Since no extensive diagenesis in R1 Lower, all logs adequately respond to interpreted facies. GR and RHOB are lower, while NPHI and DT are higher in shoal facies. Opposite responses were recorded in slightly less quality near-shoal facies. However, shoal facies in R1 Upper responses with higher RHOB and lower NPHI and DT. These logs indicate the final post-depositional characteristics affected by diagenesis. Only GR shows lower values as it was not affected by diagenesis, hence, represents critical source of information to describe depositional proxies.

Novel/Additive Information
This simple methodology helped to build the depositional maps where clear facies belt boundaries were interpreted. Accordingly, it is expected to better control petrophysical properties at inter-well locations while building 3D static model.