Hydrocarbon Detection Seabed Surveying Using Microbial-Geochemical Exploration (MGCE) Technology Can Mitigate CO2 Charge Risk Prior to Deep Water Exploration Drilling

Abstract

Objectives: Many CO2-rich natural gas accumulations have been found along the passive margins of the northern South China Sea. In deep water areas of the Pearl River Mouth Basin (PRMB), magmatic or mantle basement degassing of volatiles is the most likely major source of CO2, and upward migration is facilitated by faulting. Since seismic attributes and AvO analysis cannot distinguish CO2 from hydrocarbon gas charge, a reliable solution is required to de-risk prospects and improve probability of success prior to costly drilling in DW exploration. Methodology: MGCE seabed surveying provides, when integrated with geology & geophysics, a non-seismic reliable method to identify HC accumulations and predict their reservoir fluid properties. Microbial anomalous activity is based on a near-vertical buoyancy driven (by pressure) diffusion of C1-5 molecules through natural microfractures from accumulation to surface. Thus, MGCE is a direct indicator of a working HC system and of accumulations. Activity is reduced to lower levels or background, if traps are charged with high concentrations of non-HC gases. Results: In the PRMB, 3D seismic identified a large structural high with footwall traps. Primary targets are Oligocene sandstones charged from adjacent half-grabens. AvO indicated gas charge and a well was proposed in the area with most significant AvO effect. A MGCE survey acquired pre-drill, indicated however low background microbial values where the operator planned to drill, predicting a high risk of failure. The strong AvO anomaly proved too compelling and a costly well was drilled as planned, resulting in a “discovery” of a 25m interval with ca. 95% CO2 charge from basement. MGCE surveys successfully verified lower microbial values prior to drilling in another two deep water wells with mixed 70 and 47% CO2 charges, both also with strong AvO effects. Conclusions: 1) Exploration in settings with thin stratigraphy overlying faulted basement has a substantial risk of CO2 charge. 2) Where 3D seismic is unable to discriminate between HC gases and CO2, MGCE technology provides a proven routine which, when integrated with other geoscience information, can help to de-risk and rank prospects. A combined CO2 risk matrix will be presented. 3) MGCE is particularly beneficial where deep water drilling is expensive and minimizing risk is imperative. In many cases, proving hydrocarbon charge can improve the probability of success by a factor of two prior to drilling.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018