The Real Oil and a Bit of Gas on East Coast Reservoirs

摘要

Much early modern (1959 to 1972) and some more recent (1990) exploration in the East Coast Basin targeted Cretaceous reservoir sandstones. However these Cretaceous sandstones are generally poor reservoirs (<2 mD, and <10%) and their distribution trends are difficult to predict from outcrop and limited well data. By contrast Tertiary sandstones, especially Miocene, have good to very good poroperms, can be studied extensively in outcrop and are overlain by thick mudstone seals. In southern Hawke's Bay and Wairarapa, good sandstone reservoirs are present in basal, lower, mid-, and upper-Miocene. Although largely covered by Pliocene rocks in Central Hawke's Bay, outcrops of Paleocene to Miocene sandstones in several highs and one well intersection indicate the presence of extensive reservoir sandstones. In northern Hawke's Bay, mid-Miocene Tunanui Formation (up to 2000 m thick with about 75% nett), and the four Makaretu Sandstones (A, B, C, and D, ranging from mid- to upper-Miocene) are prospective, as well as the lower-Miocene Rere Sandstone and basal Miocene Whangara Formation. Each formation has been drilled in at least 1 well, although logs are usually poor. All are thick-bedded 'turbidites' or laminar debris flows. North of Gisborne, sandstones occur in lower, mid-, and upper-Miocene. These Miocene sandstones have average values ranging up to 26.8%/203 mD (Makaretu "D" sandstone; 16 samples) with peak values of over 30%/620 mD. Other formations average 19.7%/54 mD (Makaretu "B-C", 36 samples), and 19.3%/54 mD (Narrows Sandstone, seven samples). Samples from drillholes show values comparable to nearby outcrops of the same formation, and there is significant regional variation of reservoir quality within each formation. Pliocene coquina limestones have excellent reservoir properties; values of 35-40% and 1-4 Darcies are typical, and these formations occur extensively from northern to southern Hawke's Bay and south into Wairarapa. The challenge is to determine the subsurface distribution of these unusual shoal limestones, and to find them in structured, charged and sealed settings. None of the seven wells drilled so far targeting these formations have produced, but remember that the onshore East Coast Basin is about five times larger than onshore Taranaki. As well as these good reservoirs, the East Coast has excellent oil-prone source rocks, widespread oil and gas seeps, good structuring, formations exposed at surface, great weather, dozens of wineries, and orchards - all making it a great place to explore.