Tracing hydrocarbons migration pathway in carbonate rock in Lunnan-Tahe oilfield

Tracing hydrocarbons migration pathway is of great significance in studying hydrocarbon accumulation process and predicting hydrocarbon bearing potential of exploration targets, but it is very difficult to trace the hydrocarbons migration pathway in ancient carbonate formations due to strong heterogeneity. In this paper, a combination of geological and geochemical methods is used to discuss the hydrocarbons migration pathway in Ordovician carbonate rock of the Lunnan-Tahe Oilfield in the Tarim Basin. Deep faults and karst unconformity constitute the hydrocarbons passage framework in this area. High angle fractures caused by tectonic stress, pores and cavities formed by karstification played an important role in the connectivity and continuity of this network system. Core observation and thin-section identification revealed that the karsted holes communicating with fractures in the geologic history were the effective pathway for hydrocarbons migration in carbonate formation. The marks of hydrocarbon migration were left in these kinds of fractures and cavities, which were the direct evidence of hydrocarbon migration. The regularities of geochemical parameters of crude oil and hydrocarbon inclusion could effectively indicate the direction of hydrocarbon migration. It could be seen from the section analysis results in this paper that the features of vertical migration in a single well are apparent due to fracture communication; while in the lateral direction, the network system in which fractures and cavities are interconnected constitutes the main lateral pathway for hydrocarbons migration. Analysis results of such wells as Lunnan 63, Jiefang 127, Lunnan 39 and Lunnan 44 indicate that hydrocarbons migration direction was from bottom to top in the vertical direction and from south to north and from east to west in the lateral direction.