Outcrop-Behind Outcrop Characterization of Thin-bedded Turbidites for Improved Understanding of Analog Reservoirs: New Zealand and Gulf of Mexico

Abstract Well log, well test, and seismic data provide excellent structural and stratigraphic information on subsurface reservoirs, but may not be able to resolve small-scale vertical and lateral attributes which may control oil or gas production. Outcrops can provide this important information provided they are of sufficient areal extent. In this paper, we demonstrate the application of outcrop and behind-outcrop characterization for improved understanding of thin-bedded turbidite reservoirs in the Gulf of Mexico and elsewhere. A 200m high by 10km long coastal cliff section of the Late Miocene Mt. Messenger Formation, New Zealand consists of 200+ meters of thick-bedded turbidite fan sands and associated thin-bedded fan-fringe sandstone/siltstone, overlain by 300m of thin-bedded slope fan, levee/overbank sandstone/siltstone. This section was characterized at the ‘seismic’ to individual bed scales. Also, 47 and 105m deep holes were drilled behind the cliff face, cored, and logged through the slope fan facies with Fullbore Formation Micro Imager* and Platform Express* logs. A slope-fan model is presented which differentiates proximal levee facies composed mainly of discontinuous, erosionally-truncated Bouma Tb-c beds from distal levee/overbank facies composed of more continuous, thin-bedded strata owing to fewer truncations. An associated channel-fill facies is composed of sandstone/siltstone interbeds which become muddier upward. These three facies exhibit distinctive dip and borehole image patterns. The model demonstrates the application of outcrop-behind outcrop characterization for improved understanding of thin-bedded turbidites in analog reservoirs. For example, a permeability vs. lithofacies relationship established for the Mt. Messenger strata is common in Gulf of Mexico thin-bedded turbidites. Also, the thin-bedded levee/overbank reservoir facies of the Ram/Powell Field "L sand" in the Gulf of Mexico exhibits very similar features to those observed in outcrop and behind-outcrop cores and logs. Gas production rates of ~100 MMCFD and 9600 BCPD have been achieved from a horizontal well placed in the "L sand" proximal levee facies. In the broader sense, interactive workstation analyses of lithology, bed thickness, and dip-set groupings seen in the high resolution, behind-outcrop borehole images were used to identify depositional cycles and facies transitions based upon calibration with core and outcrop patterns. The same image analysis methods have been applied to other exploration and production wells in the Gulf of Mexico; one Neogene turbidite example is illustrated.