Reservoir Description of the Mid to Late Jurassic Laminaria Formation: Laminaria/Corallina Area, Timor Sea

Abstract

Woodside Energy Ltd. and their Joint Venture Partners recently contracted Core Laboratories to carry out a sedimentological study on the Middle to Late Jurassic succession in the AC / P8 permit, Timor Sea. The aim of the study was to provide insight into the areal distribution of reservoir facies through examination of the reservoir's vertical stacking patterns, expressed in core from key wells in the region. Particular emphasis was placed on thinly bedded sands at the top of the Laminaria Formation, which appeared to exhibit poor reservoir potential based on wireline log analysis. The study included data from a total of twelve wells, nine with core, all with complete suites of wireline log data. Conventional description of sedimentary bedforms from core was integrated with ichno fabrics, routine core analysis, wireline log data and high-resolution biostratigraphy to provide a detailed understanding of major correlation surfaces and vertical stacking patterns. This hierarchy of bedding surfaces and associated genetic packages of sediment were then utilised to assess the continuity of sands across the reservoir. subsequent dynamic simulation modeling based on this work has resulted in a significant increase in estimates of ultimate recovery factor from existing completions. The study will serve as a platform for future geological and petrophysical modeling of the Laminaria Formation. Introduction This paper reviews the methodology and results of a recently performed multi-disciplinary geological study of the Laminaria Formation. The principal aim of the study was to determine both the vertical and areal distribution of facies within the formation, and thus gain a better understanding of reservoir architecture and sandstone continuity, and thus improve estimates of recovery factor. In order to achieve this aim, the study required in-depth integration of hydraulic unitisation, sedimentology, trace fossil analysis, core analysis, core goniometry, log evaluation, and biostratigraphy. In addition, particular emphasis was placed on the occurrence and continuity of thin, high permeability sandstones in the uppermost ten to fifteen metres of the formation. Data Base Data from twelve wells was used in the study (see Figure 1). Analytical work was based on detailed sedimentological core descriptions, integrated with trace fossil interpretations, wireline log profiles, routine core analysis data and biostratigraphy. Of the twelve wells used for the study, nine were cored (Table 1). The top of the Laminaria Formation was cored in only three wells. Geological and Sequence Stratigraphic Background The Laminaria Formation is located in permit AC / P8 of the Australian Timor Sea. The formation consists of a succession of Mid to Late Jurassic sandstones intercalated with minor silty claystones and heterolithics. The sandstones are generally of good quality, and are mostly moderately to well indurated. The base of the Laminaria Formation exhibits a series of facies dislocations indicative of a regressive sequence boundary. Above this boundary, the formation can be subdivided vertically into a series of eight intervals or cycles similar to fourth-order parasequences1. These cycles, labeled A to H in order of deposition, are mostly regressive in nature, with sandstones gradually coarsening and thickening upwards, before being capped by flooding surfaces overlain by either heterolithic beds, or argillaceous, bioturbated sandstones. Most of the best quality reservoir sandstones therefore occupy the upper portions of each cycle. Geological and Sequence Stratigraphic Background The Laminaria Formation is located in permit AC / P8 of the Australian Timor Sea. The formation consists of a succession of Mid to Late Jurassic sandstones intercalated with minor silty claystones and heterolithics. The sandstones are generally of good quality, and are mostly moderately to well indurated. The base of the Laminaria Formation exhibits a series of facies dislocations indicative of a regressive sequence boundary. Above this boundary, the formation can be subdivided vertically into a series of eight intervals or cycles similar to fourth-order parasequences1. These cycles, labeled A to H in order of deposition, are mostly regressive in nature, with sandstones gradually coarsening and thickening upwards, before being capped by flooding surfaces overlain by either heterolithic beds, or argillaceous, bioturbated sandstones. Most of the best quality reservoir sandstones therefore occupy the upper portions of each cycle.