Capturing Depositional Uncertainty: Sunrise-Troubadour Giant Gas-Condensate Fields

Abstract

The Sunrise-Troubadour giant gas-condensate fields lie on a massive, broad, low-relief structure 75km long and 50km wide. The fields have a gross reservoir thickness of 80m. They are located on the north western boundary of the Australian Plate and form the basis of a proposed green-field LNG project and an Australian domestic gas development. Statistical volumetric analysis has shown that reservoir quality, distribution and Net-To-Gross ratio (NTG) are the main static uncertainties affecting Gas-Initially-In-Place (GIIP). These parameters are a function of the environment of deposition of the reservoir interval. The low average NTG in some wells (as low as 30%) indicates potential for depositional heterogeneity which can impact dynamic fluid flow through the reservoir. Depositional modelling has therefore been a major focus for field evaluation and appraisal. The reservoir interval was deposited in a marginal marine setting on a slowly subsiding, broad, low-relief shelf. The overall retrogradational character of the reservoir interval is illustrated by the upward increase in the open marine character of the interbedded shales. There are two major relative sea level falls interpreted which resulted in the deposition of incised-valley and sharp-based, forced regressive shoreface deposits. These sandbodies form the major reservoir units in the fields. Highstand and transgressive deposits are characterised by relatively thin and discontinuous sandbodies. In order to fully capture the depositional uncertainty inherent in a field of this scale which only has limited well control (six wells), three groups consisting of sedimentologists, stratigraphers and biostratigraphers have independently interpreted the core and wireline data. The result was three different depositional interpretations. From these interpretations a most likely depositional model was developed. Low and high case 3-D depositional models were generated by varying NTG and sandbody correlatibility and geometries. The adjustment of these parameters reflected the range of depositional interpretations from the three individual expert groups. These models form the basis of multi-scenario reservoir simulation studies which indicate that development options for these fields are technically robust for the range of perceived subsurface scenarios. Introduction The Sunrise-Troubadour gas-condensate fields are located 450 km northwest of Darwin on the edge of the Australian continental shelf and 50 km from the adjacent Timor Trench (Fig. 1). Together they hold in-place volumes ranging from (P90 to P10) 11.6 to 24 Tcf gas and 510 to 1055 million bbls condensate. The fields have a combined areal closure of 1000 square kilometres with a maximum vertical closure of only 180 metres. The main reservoir interval is the Bathonian Upper Plover Formation.

Identifying, Evaluating and Modelling Key Dynamic Parameters: Sunrise Troubadour Gas Condensate Fields

“…To better quantify and constrain these parameters, three of the 1997/98 appraisal wells were fully cored across the reservoir. Detailed core description, wireline log analysis and the use of analogue data were used to construct multiple 3-dimensional geological realisations using object-based geo-cellular modelling [1]. These sophisticated models were used to reevaluate in-place volumes ranges and as input to dynamic modelling.…”

“…Reservoir Quality: Low, most likely and high case geological realisations were transferred from the 3-D geo-cellular models for dynamic simulation [1]. As described, variations in reservoir architecture, quality and distribution proved to have the largest effect on field performance -affecting both in-place volumes and dynamic performance.…”

“…Smaller proportions of hydrocarbons are also contained in lower shoreface and mouthbar deposits (1-10 mD). A fuller account of the geology is covered in [1].…”

Predicting Connected Hydrocarbon Volume Uncertainty Ranges: From Static Fault Model to Dynamic Simulator

“…In 1998 a probabilistic assessment of uncertainties affecting in-place volumetrics showed the key uncertainties to be net-to-gross, reservoir properties and reservoir distribution [4] [5] (Fig. 2).…”