The Manora-Malida Fan/Delta; A Developing Geological Model for Middle Miocene Syn-Rift Deposition, Reservoir and Source Rock Characterization in the Kra Basin, Gulf of Thailand
Authors: Swire Paul , Minarwan Minarwan , Ampaiwan Tianpan , Leppard Robert , Tognini Paolo , Wilkins Michael 
Published: November 2016

The Manora Field produces oil from an Early to Mid Miocene syn-rift clastic sandstone reservoir. An integrated sedimentological model has been developed to understand the sequence stratigraphic framework and complex facies inter-relationships. Through whole core sedimentological descriptions, high resolution biostratigraphical and palaeoenvironmental breakdowns, image log analysis, seismic sequence and attribute analysis and petrographical interpretations the detailed depositional palaeoenvironments and geological history of this fan/delta in the Kra Basin are described.

High resolution core descriptions of two conventional cores from the Manora Field and the nearby Malida discovery identified fan/delta and gravity flow facies. Petrographical analyses indicated the arkosic nature of the sandstones and identified secondary diagenetic processes providing secondary enhancement of porosities. Integrated seismic sequences, biostratigraphy and detailed correlations have identified five main reservoir sandstone units that represent an early highstand prograde overlain by lowstand and transgressive units that deposited coarse cross bedded high quality reservoir sandstones over 5kms from the western controlling half graben fault. Turbidites are associated with gravity flows linked to frontal delta collapse.

Synrift faulting and fault footwall uplift contributed a large clastic input via an alluvial river system sourced from the North West. An intersecting North South trending extensional fault with an older NNW-SSE transtensional fault trend provided fault ramp systems and sediment input points into the Kra Basin, one led to the development of the Manora to Malida fan/delta. Recovery of occasionally diverse planktonic foraminifera and the presence of Scolicia traces made by a heart urchin that only lives in marine environments suggest that for periods of time basin waters were fully marine. During periods of prolonged rainfall large volumes of fresh water were discharged by the rivers reducing the salinity of the basin waters, the resulting high fluvial discharge sediment would have been transported offshore by hyperpycnal flows. Thicker sequences of coarser grained fan delta deposits are interbedded with thin turbidites (gravity flow laminated facies) and debrites (gravity flow structureless facies). The presence of thin volcanic ash bands suggests alternating periods of little sediment input and the development of anoxic conditions. Organisms would have struggled to survive with frequent changes in salinity due to variations in fresh water input and periodic isolation from the open sea.

Manora-Malida reservoirs indicate complex facies distributions; fault hanging wall fan/delta sandstones are interbedded with occasionally sub-aerially exposed and oxidized interdistributary shales. Coarse fan deposits are interbedded with fine grained laminated gravity flow deposits that represent hypopycnal processes; these developed due to frontal delta collapse and had a NE flow direction. Marine influxes are indicated by maximum flooding surfaces that generate distinct seismic markers; these are sometimes fully marine as indicated by the recovery of diverse planktonic foraminifera.

Oils in the Manora Field were sourced from organic rich lacustrine shales of Oligocene to Early Miocene age. The Saturates Gas Chromatogram of the oils show low maturity, waxy and odd-predominant alkanes, while triterpane (m/z 191) fragmentogram shows elevated C29-Norhopane and C30-Hopane peaks with both C29/C30-Hopane and C19/C23 tricyclic diterpane ratios of &lt;1.0 indicating the oils were generated from clastic source rocks with algal input. Based on GC and GCMS profiles, the Manora oils are correlated to source rock extracts of lacustrine shales in the Malida wells. Existing well penetrations indicate the presence of over 2000ft (gross thickness) of source rocks with high Total Organic Carbon (up to 5.0 wt%) and oil prone kerogen (average Hydrogen Index of 500 mg HC/gr TOC).