Objectives/Scope

Central Luconia is the most important province in the Sarawak Basin for oil and gas production from carbonate successions. It is renowned for extensive and multiple carbonate build-ups that are prolific hydrocarbon reservoirs. These carbonate buildups have high porosity, some of them exceeding 30%. A carbonate buildup is a body of locally formed yet laterally restricted carbonate sediment with topographic relief. Luconia buildups particularly have individual complexities and are more regionally variable. However, their commercial prospectivity remains a major concern.

Microporosity, prevalent throughout the Miocene carbonates, often overprints wireline logs and affects the reservoir fluid flow properties, including the ultimate recovery of hydrocarbons. The dominant lithology of the carbonate rocks is mainly limestone and dolomite.

Porous pinnacle reef carbonates are the main target for exploration in this area. Vertically stacked reservoir layers, dominated by mouldic /vuggy porosity in coarse-grained skeletal facies, are locally separated by baffle / barrier layers of tight muddier facies. Reef-bounding faults further complicate the overall reservoir distribution and connectivity, locally acting as preferential fluid pathways and possibly breaching the integrity of the overlying seal.

Several mechanisms control the hydrocarbon column heights in Luconia reefs: a) Charge access (migration focus); b) Aquifer overpressure and top seal competency; c) Thief zones interbedded in sealing lithologies; d) The well-defined carbonate build-ups have been affected by syn-depositional faulting during their growth as revealed by the seismic sections but stratigraphically.

The present study illustrates the role of advanced mud gas characterization in formation evaluation to address two main questions: is there gas in the reservoir? And, if so, is the seal working?

Methods/Procedures/Processes

This study illustrates the data logged through a cavity ring-down spectroscopic (CRDS) technique that allowed the collection of robust and reliable carbon isotopic data at the wellsite. The advantage of being at the wellsite and having continuous data logging across the whole trajectory is the higher resolution and details of the drilled formations compared to spot laboratory analyses.

The use of mud gas samples as the source of information on gas across the reservoir and seal. Mud gas extraction with controlled temperature, pressure, and mud flow is fundamental for overall correct and reliable data logging; therefore, two main systems were used to characterize the samples:

1. Chemical composition through Gas Chromatography – Flame Ionization Detection (GC-FID) from C1 to C5.

2. Carbon isotopes through Gas Chromatography – Cavity Ring-Down Spectroscopy (GC-CRDS) from C1 to C3. When drilling the seal, the short analytical cycle of C1-C2 was preferred for higher data resolution. While entering the reservoir, the analysis was extended to C3.

After data collection, many standard interpretation models from the literature were screened and adopted in inter-field correlation, and formation fluid characterization was performed while drilling via a series of Isotopic Ratios of methane, ethane, and propane, which were used to identify the fluid origin, maturity, connectivity between formations, crossing of a fault / thief zone and possible migration trends. Fluid component ratios on the gas species from Methane to Pentane were also used to identify sweet spots and potential vertical reservoir flow barriers.

Results/Observation/Conclusions

The proposed methodology was applied in two wells in Central Luconia, which resulted in one gas discovery and one oil discovery.

Gas isotope profiles in seal sediments can be integrated with the basin model to provide insight into plumbing and failure mechanisms during post-well analysis, e.g., understanding the mechanism that controls the pinnacle reef reservoirs or the sealing capacity of the overlying succession, to facilitate charge de-risking in undrilled targets.

Thanks to this information, it was possible to identify changes of isotopic trend in seal correlated to the crossing of a fault /thief zone, additionally to carefully decide the final drilling depth of the well and secure the well without further drilling complications and enabled optimized sampling for extended laboratory analyses in both wells, which lead to significant cost savings.

A trend showing increasing hydrocarbon maturity towards the oil well to gas well confirms regional maturity trends.

The isotopic ratio of Ethane and Propane also suggests a Vitrinite Reflectance (Ro) of 1-1.2 in well-B ST1 throughout the reservoir section indicating an oil window.

The data obtained is aligned with regional basin projections and legacy laboratory data.

Applications/Significance/Novelty

Carbon isotopic ratios help distinguish between biogenic and thermogenic gas and highlight the degree of mixing, which is key to establishing whether mature gas has diffused through the seal. If so, up to which stratigraphic level? Additionally, the carbon isotope logs through the seal help recognize general seal failure due to overpressure from the presence of discrete thief beds, or the absence of thermogenic charge.

In this study, we propose a quasi-real-time approach at the wellsite as an alternative to standard Isotubes collection and laboratory analysis, with an overall gain in terms of response timings and data density.

Ultimately, the carbon isotopic analysis technique based on the GeoIsotopes system provides critical analytical support, as it can be run in all types of wells (deviated, high angle) and in challenging hole conditions such as HPHT or high H2S bearing zones. While drilling, in some cases, represents the only source of formation evaluation data, in the absence of downhole data or core samples.