APGCE 2015

The VSP technique is routinely used to create seismic images near the wellbore. A seismic source is positioned below Mean Sea Level and deployed near the wellhead. The source signal is recorded by a downhole receiver that is moved to cover a large number of depth levels in the well. The upgoing and downgoing arrivals are separated during processing; the up-going wavefield is used for subsurface illumination while the down-going wavefield and multiples are excluded from the processing. The standard VSP technique using VSP upgoing wavefield gives a seismic image along the range of receiver depths and below. It does not give any information of the seismic image above the top receiver depth However, using the down-going wavefield has its own advantages, the sea surface multiple is used to obtain a VSP image of formations above the top receiver depth which is unattainable with standard VSP technique. The image obtained from the VSP multiples can have better resolution than the surface seismic data, especially for the shallower reflectors which have better signal to noise ratio. The VSP data is true amplitude, zero-phase and perfectly tied in depth and in time.

Dolomitization is a diagenetic process that can alter the rock fabric and thus affect carbonate reservoir quality. This study focuses on late diagenetic dolomite bodies in the Carboniferous host rock in Picos de Europa Province, northwest Spain. The outcrop study allows the characterization of dolomite and an evaluation of its link with sedimentary lithologies and structures.

Field observations have shown that dolomite bodies of elongated geometries and varying sizes are present, with the principal length axes parallel to fault planes. The dolomite bodies have higher porosity than the limestone host rock. The geobodies are elongated in the direction of the main faults and their length/width ratio is about 2. Geobodies hosted in different formations vary significantly in dimension, implying that large dolomite bodies are favoured in zones with large fluid input and accumulation to allow for extensive chemical reaction.

This study has shown that the dimension and distribution of the dolomite bodies are inherently linked to a combination of factors including the original lithology of the host limestone, pre-dolomitization diagenetic history, and structural elements. This study has also provided an understanding of the diagenetic history in the study area, as well as a dataset of dolomite body dimensions, relevant for subsurface models.

Imaging below shallow gas bodies is one geophysical challenge encountered in offshore Southeast Asia basins. There are two key components in addressing this challenge: 1) the derivation of an accurate, high-resolution, geologically consistent velocity model and 2) compensating for absorption of the seismic signal as it propagates through the gas bodies.

An accurate Earth model in the overburden will not only improve imaging in the shallow section but will also facilitate subsequent model building and improve deeper reservoir-level imaging. Full-waveform inversion (FWI) velocity model building operates in the data domain and presents the capability of creating a high-resolution velocity model through a wavefield-consistent solution.

Q tomography and Q imaging address the Earth’s attenuation effects by first deriving a spatially and temporally variant 3D interval 1/Q model using ray-based reflection tomography techniques and then incorporating this model within a migration operator.

In this study, we present the application of FWI and Q tomography to a shallow-water, shallow-gas dataset from offshore Southeast Asia. We demonstrate the successful ability of FWI and Q tomography to resolve the low P-wave velocities and high attenuation of shallow gas bodies and subsequently compensate for the complex kinematics and absorption during depth migration.

The 4C 3D OBC survey’s main objective is to record both compressional and shear wave information, whereby shear waves are not sensitive to fluid effects thus allowing the seismic signal to propagate below the shallow gas area. This paper aims to demonstrate the improvements in structural interpretation and reservoir characterization through integrated evaluation of 4C 3D OBC PP & PS APSDM datasets in S Field, in the Malay Basin, offshore Peninsular Malaysia.

Although the PS dataset has lower resolution than the PP dataset, it does contain higher signal-to-noise ratio compared to the PP below the shallow gas. It is crucial that both datasets are integrated when conducting structural interpretation.

The 4C 3D OBC has been successful in recovering travel time delay and amplitude masking below shallow gas. In addition, the PP-PS Joint Inversion was able to produce porosity volumes that closely match with well porosities. Consequently, the usage of 4C 3D OBC PP and PS dataset plays an important role in providing high fidelity structural interpretation and reservoir characterization of Field S, providing higher confidence to the Asset Team in terms of hydrocarbon resource assessment and well placement.

Sarawak is located on the northern edge of Sundaland in NW Borneo. West and Central Sarawak include parts of the Kuching and Sibu Zones. These contain remnants of several sedimentary basins with ages from Triassic to Cenozoic. New light mineral, heavy mineral and U-Pb detrital zircon ages show differences in provenance reflecting the tectonic evolution of the region.

This project presents new Ar-Ar ages from metamorphic rocks in West Sarawak that require a revision of tectonic models, as well as new U-Pb zircon ages, light and heavy mineral compositions of sedimentary rocks in West and Central Sarawak. A revised stratigraphy has been developed in this study. The sedimentary successions in are dated as Triassic to Cenozoic in West Sarawak and Late Cretaceous to Cenozoic in Central Sarawak

Our data show that the area of the Kuching (West Sarawak) and Sibu (Central Sarawak) Zones were connected with SW Borneo and Sundaland from the Cretaceous onwards. The Cretaceous and Cenozoic sedimentary basins were sourced by alternations of Schwaner Mountains and Malay Tin Belt rocks and contemporaneous volcanic activity.

Block 2F resides in the Bunguran Trough, an intra-continental basin located in the deepwater setting of the Rajang Delta, offshore Sarawak at the centre of the South China Sea. The trough evolved as a tectonically-induced sag basin. Its oldest known stratigraphy is formed by shelfal clastics of the Early Miocene Cycles I/II, now buried beyond a depth of 6000m. The Late Miocene Cycle V section is formed by some 3000m of slope and toe-of-slope deposits, overlain by distal and muddy sediments of the younger Plio-Pleistocene sedimentary succession with turbiditic fairways forming the main objectives in the current exploration campaign. Due to limited well information and poor seismic quality, the complete post-Cycle I sequence stratigraphic interpretation and GDE study of the area is not possible and can only be meaningfully conducted in the younger Miocene to Plio-Pleistocene section. These GDE maps include predictive models for reservoir, source, and seal distributions, in addition to identify and predict key turbidite sand sequences and trends, as the most significant geological risk in the deepwater province offshore Sarawak is the presence of effective reservoir.

This integrated study has shown that it is necessary to combine seismic, CSEM, seabed geochemistry and microbial data when exploring for hydrocarbons in frontier regions to avoid the risk of over-interpreting any individual data set. The spatial pattern of the MPOG result is inconsistent with CSEM shallow-depth information and may be a reflection of sampling quality. We recommend that coincidentally-located seismic, CSEM and seabed geochemistry and MPOG surveys should be adopted to reduce exploration risk and improve the success rate of finding hydrocarbon-filled reservoirs in such frontier regions

Jelawat-1ST1 was a wildcat exploration well drilled by Amerada Hess (Malaysia) Limited in 2004 testing an anticlinal feature of the Bunguran Trough Fold-Thrust Belt located in the current JX Nippon operated Block 2F. Late Miocene turbidites of Cycle V sequence with high amplitude anomalies were the deepest targeted objectives of Jelawat-1ST1 well; however, only tight reservoirs with minor gas shows were encountered. No other effective reservoir intervals were encountered in the overlying shallower section, which is predominantly made up of silty sands and shales. In 2011, a paper presented by Ong et al. (2011) based on the outcomes of Fluid Inclusion Stratigraphy (FIS) analysis suggested that the Jelawat-1ST1 well (annotated as “Well A” in the paper) showed sparse low gas anomalies throughout the studied interval with no visible liquid hydrocarbon inclusions in the analysed thin sections, albeit their 3D basin modelling study suggesting adequate hydrocarbon generation and migration to any potential entrapments. However, it was concluded that late deposition of the regional top seal is ineffective to retain any economical hydrocarbon accumulations.

The sand rich, deep water facies association of the West Crocker Formation has been ascribed to four principal facies elements viz a metre scale, thick, massive sandstone facies; thin tabular, planar to ripple laminated sandstone — mudstone facies; sand or mud rich debris flow with occasional slump facies; and lastly plane laminated mudstone facies. The predominant facies, thick massive sandstone facies, which is apparently massive, presents significant variability. Six sub facies of the massive sandstone facies attributed to different processes of occurrence are:

1. Massive, structureless sandstones
2. Sandstones with faint, crude planar laminations or cross stratifications
3. Sandstones with random floating clasts
4. Sandstones with fluid escape and dish structures
5. Sandstones with convolute lamination
6. Sandstones with rugose upper surface and upward sand injectites.

The aim of this article is to exemplify that simplified, apparently massive, thick sandstone units have different processes of occurrence.

All the details and distinctive observed sedimentary structures, bed thickness, grain size, grain and matrix composition, all allow the finer distinction of these massive to apparently massive sandstone associated with gravity flow deposits from turbulent to truly laminar flow processes in aspect of process sedimentology for sand rich deep water depositional system.

An issue of importance for the petroleum industry is where clastic sediments came from, when, what type they are, and where they went. This is also important in regional tectonic studies; the geological record is dominated by marine deposits, but the history of land is commonly an inference from gaps and unconformities. Studies of exposed basement rocks and the provenance of clastic sediments can provide insights into areas of uplift, sediment sources, and sediment supply routes which are especially valuable in geologically dynamic regions like SE Asia.