Evaluation of Low-Resistivity-Pay Deepwater Turbidites Using Constrained Thin-Bed Petrophysical Analysis

Abstract

A novel methodology has been developed to evaluate low resistivity pay reservoirs in very thin, laminated sand-shale sequences. This technique combines information from both high and low resolution logs to model the petrophysical properties of thin bedded formations. The result is a set of modeled high resolution logs that can be used to yield more accurate estimations of porosity, water saturation and permeability. A high resolution and a low resolution log response are selected and binned into discrete lithofacies. The average low resolution log responses are statistically calculated for each lithofacies. An initial model is generated by using the high resolution lithology index to select the corresponding low resolution response for each lithofacies, resulting in a set of high resolution logs for each input log. By applying a corresponding vertical response filter to the high resolution log, a synthetic log is created which can be compared to the original. A minimization technique is used to adjust the high resolution responses such that the synthetic and actual logs agree. Since there are an unlimited number of high resolution logs which could result in the same synthetic response, the process is constrained using limits defined for each lithofacies. Evaluation of hydrocarbons in place is then performed on the resulting model. This paper documents examples in deep-water turbidite reservoirs where thin beds on the scale of a few centimeters were evaluated using this technique. The observations were consistent with core data where this was available. Thin bedded silt and shale laminae had a disproportionate effect on standard resolution measurements due to the abundance of montmorillonite clay which caused elevated neutron and suppressed resistivity responses. The constrained thin-bed analysis yielded considerable enhancement of hydrocarbons in place compared with the evaluation using only standard resolution measurements. Thin bedded laminated reservoirs occur in many hydrocarbon provinces. The technique described in this paper provides an alternative method for evaluating these formations, helping to reduce reserves uncertainty during a time where accurate reserves evaluation continues to receive high attention. Introduction Formation evaluation in thin shale-sand sequences has been a challenge. Thinly laminated pay zones are frequently overlooked when using conventional formation evaluation. Identification of deep-water turbidite sequences and their productivity are affected by clay characteristics, mineralogy, the unconsolidated nature of the sediments and low resistivity contrast pays. Extensive efforts, involving both monetary and human resources have been made in the past decade for identification and enhanced evaluation of such thin shaly-sand sequences. Motivated by the initial success of identifying and evaluating the pay zones encountered in the deep water exploration block of the Krishna-Godavari Basin, east coast of India, shown in Figure 1, the major challenges were to extend the exploration campaign to the adjacent deep sea areas and to estimate the reserves associated with the thin bedded shaly sands. Evaluation of thin beds that are less than the resolution of standard logs was the greatest challenge. Pay zones were identified by the integration of high-resolution logs and image data with seismic. Inverse modeling the thin sand-silt-shale formations using total clay content from X-ray diffraction (XRD) analysis of cores, it was possible to estimate a realistic hydrocarbon saturation within the pay zones. A novel technique of enhancing the resolution of logs to the level of microresistivity imaging tools helped us in the estimation of log response in such thinly laminated pay zones. The following sections describe the geological setting of our case study, the methodology of our new technique, the results and summary and conclusions.

Heterolithic Thin Bedded Reservoirs Volumetric Estimation, Quantification and High-Resolution 3D Modelling

“…The heterolithic reservoirs contribution is important to unlock the field potential for development optimization. Most of the production fields in Malaysia basin and around the globe are from conventional reservoir associated with heterolithic thin bedded (HTB) [1][2][3][4][5][6]. Heterolithic Reservoir description depending on scale of thin bedded, heterogeneous, or laminated pay, which refer to the formation with two or more sand members that non-communicating, each probably with distinct relative and specific permeability characteristics as explain by Henderson et al, [7], Kantaatmadja et al., [9], Nayagawa et al, [12], Stromberg et al, [15].…”

A New Approach for the Realistic Evaluation of Very Thin Reservoirs of Krishna Godavari Basin, East Coast, India

“…Identification and location of bed boundaries were nicely resolved as bed thicknesses (~ 1 cm) were comparable with the vertical resolution (0.5 cm) of the image tool. Similar approach (Bastia et al, 2007), can not be extended for the thin bed evaluation using oil base mud image (OBMI) tool, where vertical resolution is six times inferior as compared to FMI tool. There are many productive reservoirs whose beds are too thin to be defined by high resolution log.…”