AAPG Middle East Region Geoscience Technology Workshop

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The Amin-Miqrat Boundary in Northern Oman: Evidence of Unconformity Development and Significance for Producing Fields

Abstract

The Amin Formation, the lowermost unit of the Haima Supergroup in Oman, is one of the main gas and condensate producing intervals in the country. It is predominantly sealed by the immediately overlying Miqrat Formation. These formations were deposited during the Cambrian period in the two salt basins of northern Oman, the Fahud and Ghaba Salt Basins. In terms of gross depositional environment, the Amin and Miqrat are quite similar. The Amin was predominantly deposited by braided fluvial systems, but features a significant amount of reworking in aeolian and dry sabkha environments in its upper portions (Amin-S30 in the subsurface). The Miqrat was also deposited by a continental alluvial system, but with more widespread sabkha development and even playa formation locally. Despite these depositional similarities, the two formations are very distinct in wireline logs. The Amin-S30 is characterised by a very low gamma response (typically <50 gAPI), whereas the lower part of the Miqrat has a much higher gamma values (usually >100 API). Part of the reason for this is lithological – the Lower Miqrat contains a lot more fine-grained and clay-rich lithologies – and part is compositional, as sandstones in the Miqrat tend to be much richer in potassium feldspar than those of the Amin. Thus, there must be a significant change in sediment provenance across the Amin-Miqrat boundary. In outcrop the uppermost part of Amin-S30 shows clear evidence of marine influence, with metre-scale progradational parasequences and bedding planes featuring horizontal burrows, arthropod trackways and ‘elephant skin’ textures of algal origin. The evidence of marine influence appears to increase upwards, until the boundary with the Miqrat is reached. Although there is no biostratigraphic age control in the sandstone dominated S30, a marine flooding surface referred to as Cm20 has been documented in this part of the stratigraphy elsewhere in the Arabian Plate. The base of the Amin-S30 (i.e. the top Amin-S20) provides a convenient datum on which to flatten subsurface correlations of the Amin. When this is done tens of metres scale variations in the thickness of the preserved S30 are observed from one well to the next in certain fields. Individual cycles with the S30 do not however show significant thickness changes, unless truncated by the overlying Miqrat. The most likely explanation for this is that an uplift and peneplanation event occurred after the deposition of the Amin but prior to Miqrat deposition. The uplift was most likely a localised effect related to movement of the underlying Ara salt, and the peneplanation a result of the marine transgression that began just before the end of Amin deposition. The reservoir properties of the Amin show significant vertical variations, with the relatively clean and mature sandstones of the S30 typically being much better than the immature and heterogeneous sandstones of the S20. Thus, wells where the S30 is thin tend to perform less well than those with a thickly preserved S30. In some fields, poorly producing wells with little S30 tend to be clustered towards the crest of the present-day structure, suggesting that early salt-related uplift of the Amin was a localised effect focused in these areas.