Application of Saturation Height Function in Deep Water Turbidite Gas Sands, Offshore Northwestern Myanmar and its Implication

Abstract

The Shwe gas field discovered in 2003 is located offshore northwestern Myanmar. The gas reservoirs are Late Pliocene turbidite sandstones deposited between 2,900 and 3,300 meters sub-sea with net sand thickness up to 75 meters. Two distinctive types of reservoirs were defined in the discovery; one with thinly bedded sand deposited in a levee system (G3), and the other with massive sand deposited in a lobe system (G5). In general, both sands show fair to good reservoir qualities, but two key features have caused an obstacle to accurate reservoir evaluation; 1) several water intervals intercepting a single gas gradient from the upper G3 to the lower G5 sands, which may show shallow free water levels (FWL), and 2) the depressed resistivity in the thinly bedded G3 sand, resulting in high water saturation even in water-free gas sampled intervals. This study illustrates how we defined the characteristics of the water intervals and recovered the hydrocarbon saturation by integrating well logs and core analysis, and discusses their implications in deep water turbidite sands. Intensive core analysis was conducted from 200 plugs including electrical properties, capillary pressure and relative permeability measurements. 4 reservoir facies were defined based on poro-perm and irreducible water saturation, and saturation height function (SHF) was generated on each reservoir facies. FWL was identified by matching the SHF to the log-derived shaly sand water saturation within the massive G5 sand, then which has been applied to other intervals. Through this process, it is confirmed that the intercepting water intervals are localized one rather than defining each FWL, and the log-derived water saturation in G3 has been overestimated due to thin bed effect. Especially, with support of seismic interpretation, production data and geological analogy, the localized waters are recognized as perched water by post erosional channel, water lag in the bottom of turbidite sand by erosional process, and isolated water within thinly bedded sand. Application of SHF to the thinly bedded reservoir, G3 has successfully resulted in recovering the hydrocarbon pore volume up to ~50%, compared to the log-derived one, where production data also shows water-free gas flow up to 15mmscfd. To summarize, the application of SHF in this deep water turbidite sand, indicates that 1) localized water could exist regardless of the depth structure due to its depositional process, and 2) water-free commercial hydrocarbon can be recovered even in high water saturation interval of the thinly bedded reservoir. It strongly suggests that water distributions and thinly bedded reservoir observed in deep water turbidite sand should be clearly justified in exploration and appraisal period, for reserve estimation and also development plan.

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