Quantifying Gas Hydrate Deposits- Implications for Petroleum Systems and Secondary Target and Seal Assessment in Deepwater Myanmar

Abstract

Buried gas hydrates are usually not considered when planning and executing deepwater oil and gas developments. In most deepwater areas, located over active petroleum systems that can “hotel” migrating gas in the form of gas hydrate, the gas hydrates usually do not pose any serious issue for well planning or field development. In these cases, a weak effort at identification and avoidance has been sufficient. However, there are a few places in the world where gas hydrate deposits have interfered with or fundamentally changed oil and gas development plans. These places include offshore Malaysian Borneo, where gas hydrates are concentrated in the shallow soils, and Colombia, where gas hydrates are in close proximity to, and may seal potential secondary commercial drilling targets. These critical gas hydrate issues are likely present in the geological setting in deepwater Myanmar, where organic rich sediments subduct beneath the Burma platelet, generating large amounts of biogenic gas that migrates into thick series of sand-prone abyssal north-south trending Brahmaputra fan deposits and east-west Highland channel deposits that intercalate in both the shallow and upper exploration sections. Identifying and quantifying gas hydrate reservoirs is still underdeveloped within the spectrum of E&P technology because gas hydrates usually do not need to be analyzed, but research and scientific exploration efforts have developed a set of tools and exploration context that can be used to quantify and identify gas hydrate thickness and saturation. A key difference, in contrast to gas exploration, is that there is a linear relationship between seismic amplitude and gas hydrate saturation. Another key difference is that gas hydrate deposits can often have fast (gas hydrate) and slow (gas-filled) seismic responses within the same seismic thin bed, or as a thin bed response within a thicker sand body. These key seismic responses can be exploited using adaptations of AVO and spectral decomposition analysis for gas hydrate to answer questions such as: How thick is the effective gas hydrate seal over this gas deposit? What is the saturation of the gas hydrate seal? Is there an ephemeral seal that will be destroyed when producing shallow secondary targets? In the Eastern Gulf of Mexico there are sand-rich abyssal fan deposits analogous to the Brahmaputra channel deposits that in both instances span the base of gas hydrate stability. Mapping the gas hydrate and gas response over flat-lying unconstrained channel deposits provides a laboratory for testing seismic response of gas hydrate and gas fill within the same channel body where the channel body crosses the base of gas hydrate stability in a zone of gas flux from deeper in the section. Spectral decomposition has been used as a direct hydrocarbon indicator (DHI) for conventional exploration, but in most cases, only empirically where the frequency response of known pay is used to find other pay. In the test case in the Eastern Gulf of Mexico, spectral decomposition is used as a true independent direct hydrocarbon indicator when paired with the thickness concepts of Partyka (1999) and where, in the test case, well behaved channel morphology enters and exits the zone of hydrocarbon flux. Pseudo-spectral thickness concepts can then be used to delineate gas hydrate filled channels from gas filled channels. These techniques can also be used to isolate and determine the thickness of a gas hydrate thin bed response that may only fill a portion of a thicker sand body. These techniques, plus other seismic thin bed responses that can be isolated with seismic attributes such as envelope derivatives, can be used to quantify gas hydrate presence, thickness, and saturations in gas hydrate deposits in deepwater Myanmar and can help determine secondary target and seal effectiveness where gas hydrate deposits are proximal to potential secondary gas targets.

AAPG Datapages/Search and Discovery Article #90336 ©2018 AAPG Asia Pacific Region, The 4th AAPG/EAGE/MGS Myanmar Oil and Gas Conference, Myanmar: A Global Oil and Gas Hotspot: Unleashing the Petroleum Systems Potential, Yangon, Myanmar, November 13-15, 2018