Petroleum geochemistry and depositional setting of lacustrine source rocks

Abstract

Lacustrine sequences are increasingly being recognised as hosts for commercial reserves of petroleum. Petroleum source rocks in lacustrine sequences have organic carbon contents ranging from <1% to >20% and kerogen types range from Type I to Type III. The organic matter can be of land plant, algal or bacterial origin. Most undegraded lacustrine oils are extremely paraffinic and waxy, but a few are asphaltic and rich in sulphur. The crude oils and source rocks contain a wide diversity of biological marker compounds reflecting different source inputs. The diversity of chemistries displayed by lacustrine source rocks and crude oils reflect the wide variety of organic source materials and depositional conditions operative in lakes. Lacustrine sequences preserved in the geological record represent sediments deposited in fluvial lakes on humid flood plains and tectonic lakes in humid, semi-arid and arid environments. Preservation of organic matter in lakes is dependent on the formation of anoxic or micro-oxic bottom waters. This is favoured by stratification of lake waters brought about by temperature and salinity contrasts. Organic material preserved in oligotrophic lakes is largely of land plant origin which has been extensively modified by bacteria under mildly oxidizing conditions and supplemented by the bacterial remains. Source rocks of this type are found in: (1) fluvial - lacustrine settings (e.g. Jurassic of Eromanga Basin, Australia) and (2) the oligotrophic phase of larger tectonic lakes (e.g. early stages of Green River Formation, Uinta Basin USA). As these freshwater lakes become productive, algal remains are contributed to the sediment and anoxic bottom waters can become more firmly established. The green algae Pediastrum and Botryococcus appear to be particularly important although land plant detritus is still significant (e.g. Cretaceous source rocks, Songliao Basin, China). Saline lakes conditions tend to restrict the diversity of the source biota although preservation may be enhanced through anoxia brought about by salinity stratification and inhibition of bacterial decay (e.g. later stages Green River Formation Uinta Basin, USA). Sulphate-rich, and hypersaline lacustrine systems result in the formation of sulphur-rich sources and oils (e.g. Jianghan Basin, China). Because of the diversity of organic matter types, maturation criteria for hydrocarbon generation in lacustrine sequences also vary. However in most cases, higher maturation levels are required for significant hydrocarbon generation. At present, it is uncertain the extent to which algal and bacterial organic matter contribute to the source for the waxy hydrocarbons which are characteristic of lacustrine oils. Organic-rich rocks formed in large freshwater and saline lakes are the most productive type of lacustrine source rock, although sources formed in lakes of flood plains and deltas are also widespread.