3D Close-the-Loop Method Based on Probabilistic Seismic Inversion
Authors: Carpenter Chris 
Published: April 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 18913, “3D Close the Loop Using Probabilistic Seismic Inversion for a Gas  Field in the Carnarvon Basin, Australia,” by Shilpi Srivastava, Jeroen Goudswaard, Sito Busman, and Justin Ugbo, Shell, prepared for the 2016 International Petroleum Technology Conference, Bangkok, Thailand, 14–16 November. The paper has not been peer reviewed. Copyright 2016 International Petroleum Technology Conference. Reproduced by permission.

The paper discusses an approach for predicting the lateral variation of net to gross (NTG) by use of 3D probabilistic seismic inversion. The goal is to define and understand the distribution of sands and shales on the basis of seismic reflection data. The modeling and inversion are supported by the good quality of seismic data. This study underpins the benefits of seismically constrained reservoir modeling. The use of probabilistic inversion to map geological features is a new insight in the applicability of this methodology.

Introduction

The study field is located in the Carnarvon Basin offshore western Australia. The field was appraised with one well, which has added incremental volumes to existing nearby discoveries. Seismically constrained reservoir-model building creates models that are constrained by, and matched to, geological concepts and seismic amplitudes and travel times. The complete paper shows an approach using 3D probabilistic seismic inversion to predict the lateral variation in NTG over a proven discovery. This, in turn, can be used to update the static-model properties, improving on the first-pass static-model build. Additionally, the inversion will aid in the understanding of some poorly understood seismic expressions, especially in the south of the field. In this area, the authors noticed dim amplitudes on the amplitude maps extracted from the seismic data, where the static model predicts the presence of the reservoir.

Field

The field is located in the Carnarvon Basin. The discovery was appraised by one well, which penetrated a thin sand layer of interdistributary bay reservoir facies (referred to as Sand 1) and a thicker distributary channel facies (referred to as Sand 2). The overburden comprises basinal shales and marls.

The main steps in the 3D close-the-loop work flow are

The rock-property-trend models are derived using the available vertical wells in or near the field. A “rock model” is a set of equations that honors well data and links the petrophysical properties such as NTG and porosity with rock acoustic properties.

A qualitative 3D check-the-loop (CTL) step is carried out. This involves the initial fit of the static model with the seismic data by forward modeling the synthetic seismic computed from the initial static model before inversion. The quality-control step that follows aims to detect mismatches between seismic and synthetic data.

Finally, constrain the reservoir model by use of the seismic data by carrying out 3D probabilistic inversion of the key uncertain rock property.