Abstract
Net to gross (NTG) is an important parameter in evaluating the volumes of hydrocarbon in place (VHIP) in reservoirs, and proper evaluation of this parameter will lead to significant accuracy in the estimated reserves. The use of conventional petrophysical log evaluations often does not provide sufficient resolution for net sand analysis of reservoirs, especially in laminated reservoir rocks. The uncertainties associated with log-derived net-to-gross estimates arise from the petrophysical shale volume (V sh) cutoffs used in deriving the net sand count over a reservoir interval. One way of improving the net-to-gross estimates is by using a model that calibrates log-derived net to gross to a core equivalent, which is the more accurate representation of net sand counts. In this study, a model for calibrating log-derived net to gross to a core equivalent based on genetic units and facies associations of reservoir rocks in three wells (wells 7, 36, and K008) was established. Ultraviolet core photographs show a good contrast between hydrocarbon-stained sandstones and shales, and combining it with white light-slabbed core images facilitated a manual net sand count of the core photographs on a bed-by-bed basis. Petrophysical shale volume (V sh) cutoff derived from two volumes of shale indicators was applied to generate net sand counts, which were used to get log-derived net-to-gross values. Then, the net to gross from core images and petrophysical clay volume (V sh) analysis were compared by facies associations, and this comparison yielded a reliable core-calibrated net-to-gross model, which reduces the uncertainties in net-to-gross values estimated from V sh cutoffs. The results show that for the distributary channel sands, net to gross derived using V sh cutoff results in an underestimation of net to gross by about 6–10 % when compared with the core-calibrated net to gross, while for the upper shoreface units, V sh cutoff overestimates net to gross by about 7–10 % when compared to its core-calibrated equivalent.
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References
Adegoke AK (2012) Sequence stratigraphy of some middle to late Miocene sediments, coastal swamp and western offshore Niger Delta. Int J Sci Technol 2(1):18–27
Ajaegwu NE, Odoh BI, Akpunonu EO, Obiadi II, Anakwuba EK (2012) Late Miocene to early Pliocene palynostratigraphy and palaeoenvironments of ANE-1 well, eastern Niger Delta, Nigeria. J Min Geol 48(1):31–43
Avbovbo AA (1978) Tertiary lithostratigraphy of Niger Delta. Am Assoc Pet Geol (AAPG) Bull 62:295–307
Doust, D.M., and Omatsola, E., 1990. Niger Delta. In: Edwards JD, Santogrossi PA (eds) Divergent/passive margin basins. American Association of Petroleum Geologists (AAPG) Memoir 48:239–248
Evamy, B.D., Harembourne, J., Kamerling, P., Knaap, W.A., Molloy, F.A. and Rowlands, P.H., 1978. Hydrocarbon habitat of Tertiary Niger Delta: American Association of Petroleum Geologists (AAPG) Bulletin 62(1):1–39
Henderson, K., Rose, H. and Winter, R., 2010 Identifying and quantifying thin bedded pay (part A): log characteristics and reservoir quality. Society of Petroleum Engineers. Paper number 133534-MS, 2010
Nwajide CS (2013) Geology of Nigeria’s sedimentary basins. CSS Bookshops Limited, Lagos 565p
Tuttle, M.L.W., Brownfield, M.E., and Charpentier, R.R., 1999 7 Tertiary Niger Delta (Akata-Agbada) Petroleum system, Niger Delta Province, Cameroon and Equatorial Guniea, Africa. U.S Geological Survey Open File Report 99-50 H, pp.1–13.
Weber, K.J and Daukoru, E., 1975. Petroleum geology of Niger Delta. 9th World Petroleum Congress Proceedings, v.2, p 209–221
Acknowledgments
The authors wish to acknowledge the management of Shell Petroleum Development Company (SPDC), Port Harcourt, especially the leadership team of the corporate petroleum engineering discipline (CPE) for the permission to publish this work. This study benefitted immensely from contributions of Dr. Oden Ezimah, the Exploration Petrophysicist of the SPDC.
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Anyiam, O.A., Aghara, K.I., Mode, A. et al. Hydrocarbon volume optimization using improved net-to-gross estimates through core-log calibration of the “Akar field,” Niger Delta. Arab J Geosci 9, 547 (2016). https://doi.org/10.1007/s12517-016-2564-x
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DOI: https://doi.org/10.1007/s12517-016-2564-x