Abstract

The Lelet Limestone of New Ireland is a massive early to middle Miocene limestone complex of biogenic origin. It is up to 1400 m thick on land, consisting mostly of reefal deposits of varying contributions from corals and algae. A significant input of organic matter is characteristic of certain horizons and so is pyrite, implying anoxic conditions possibly on microenvironmental scale. Organic- carbon content in the Lelet Limestone organic concentrates ranges from 3.24 to 12.06%. TOC range from 0.05 to 0.97% (excluding coal inclusions of the Matakan Unit). Hydrogen content is between 2 and 3%, Sulphur content varies and is directly related to pyrite content and organic-carbon concentrations. Oxygen is medium to high, reflecting on high humic- acid content, in itself the result of extensive oxidation in the weathering zone. H/C atomic ratios are around 1.5.

Limestones have in the past been regarded solely as reservoirs for hydrocarbons. The present study shows that limestones of biogenic origin such as reefs may also be petroleum source rocks. Previous studies on organic matter in limestones have shown that hydrocarbons are found in higher concentrations in limestones than in shales. Overall organic matter however is generally lower in the former. Some parts of the Lelet limestones have undergone little diagenesis, as evident from the presence of aragonite. Red algae and possibly bryozoans because of their intricate structure may create microenvironmental anoxic conditions where organic matter is trapped, concentrated and preserved. The presence of high pyrite concentrations in parts of the limestone is in itself an indicator of anoxic conditions and sufficient organic matter for bacterial sulphate reduction. The algal-reef facies of the Lelet Limestone contains usually above 1% total organic matter in outcrop samples. It is likely that deep-seated rocks of this sequence contain much higher concentrations of organic matter and may therefore be potential hydrocarbon source rocks. The Matakan Unit within the Lelet is an organic-rich back-reef lagoonal deposit with coal inclusions. The latter are of cannel and vitric type, where the channel parts have generated hydrocarbons at low saturation, less than 0.5% Ro. This unit if proven to be widespread adds significantly to the hydrocarbon potential of the Lelet Limestone sequence.