Improved Petrology & Petrophysics Characterization of Complex Siliciclastic Reservoirs through Integrated Logs Analysis- Case Study in South China Sea
Authors: Cai Jun , Guo Shusheng , Gao Yongde , Li Hui Geng , Xian Chenggang 
Published: May 2016

Petrology attributes of reservoir rock are fundamental elements resulting varied petrophysical responses, and controlling reservoir flowing performance. This paper presents a workflow characterizing complex sand-shale sequenced reservoir with a case study in South China Sea. An integrated approach of integrating elemental neutron capature spectroscopy, formation micro-resistivity image and nuclear magnetic resonance (NMR) data are used to characterize reservoir petrology attributes of mineral composition, grain size, sorting and layering fabric; with improved understanding of state dependency problems, such information is of more use in petrophysical evaluation and reservoir producibility simulation. The information can also be integrated into further geological study to improve the certainty of model. Such an integrated approach is proven an effective way to reveal the reservoir intrinsic properties, better to understand interrelationship of sedimentary model, petrology and petrophysics of target formation, and more accurately to delineate reservoir.

The marginal reservoir of the case study presents numerous challenges such as low porosity/low permeability, complex porosity-permeability relationship, high apparent water saturation that may or may not produce water, and thin laminations. Due to the nature of these complexities, different reservoir zones with very similar looking in conventional logs behaved totally different production performance from well testing results. This created difficulties for routine petrophysical analysis which often resulted in under-estimated or overestimated reservoir zones without consistency. By integrating multi logging analysis skills with improved methods to characterize sedimentary petrology & intrinsic petrophysical properties of reservoirs, the approach has been demonstrated as an effective way of well understanding the reservoir and evaluating productivity with significantly improved accuracy. Accurate pay zone characterization gained immediate economic impacts on offshore operation. More importantly, the reservoir knowledge gathered is definitely beneficial for further appraisal and future field development planning.

Introduction

Tracing back to the 1950s when G.E. Archie introduced the term of "Petrophysics", this petrophysics founder had noted that the specific Rock Type, whose parts have deposited under similar conditions and undergone similar processes, would have certain effective pore-size distributions and particular capillary pressure curves, hence controlled porosity and related permeability as well as water saturation [Archie, 1950]. Rock type, which can be described in petrology with attributes such as composite minerals, grain size, sorting and fabric et al, fundamentally resulted in varying petrophysics property. Well understanding the rock petrology and petrophysics properties through the well has penetrated, is also important to geologist and reservoir engineer, to in-depth study the sedimentary facies and flow units, so as to build an accurate geological model and flow model.

Conventionally, thorough delineation of the rock with characterizing the rock attributes, such as rock texture and sedimentary structure, has to resort to drilling full-bore cores, although sidewall cores and drilling cuttings are also helpful in some extents; conventional lithology logs such as neutron porosity, formation bulk density and natural gamma ray, are more often used to evaluate and correlate the formation in a continuous manner, but most of time it's difficult or not enough to reveal above rock intrinsic attributes, which result in distinguished reservoir flow performance in the root. These conventional logs may look similar for various reservoirs, as like the case presented in this paper.