Shale oil occurring between salt intervals in the Dongpu Depression, Bohai Bay Basin, China

Abstract

The ‘saline mudstone’ sequence of the Paleogene Shahejie Formation in the Dongpu Depression of Bohai Bay Basin is a major source rock for conventional hydrocarbons. However, shale oil, having a density of ~0.9g/cm3 and a viscosity of ~21mPa·s, has also been discovered and is being produced from these mudstones. To better understand the occurrence of such shale oil, geochemical petrological and mineralogical investigations were carried out on a suite of saline mudstone samples by using Rock-Eval, X-ray diffraction and SEM.The Shahejie Formation samples analyzed in the current study are characterized by type II kerogen with lesser amounts of types I and III, such that petroleum generation ranges from ‘fair’ to ‘excellent’ in general terms. With the exception of Es1 (uppermost member), which is immature to marginally mature for oil generation, the sequence studied ranges from ‘mature for oil generation’ to ‘mature for wet gas generation’. On the basis of vitrinite reflectance data, the most prospective depths for shale oil/condensate generation are between ~2300 and 4000m depth in area studied. The saline mudstones comprise mainly clay minerals, followed by carbonates, quartz and feldspar. Based solely on the ‘brittle mineral index’, the amounts of carbonate and quartz in many of the samples lead to a similar, or greater suitability for hydraulic fracturing than for many Haynesville, Woodford and upper Bakken shales. SEM studies show that abundant micro-fractures commonly occur in the mudstones and these include tectonic fractures, bedding plane fractures, shrinkage fractures, and over-pressured fractures. For the Es3M (middle third member) unit, shale oil prospectivity is likely to be enhanced around the towns of WL (Wenliu), LT (Liutun) and HBZ (Hubuzhai), due to the increased frequency of tectonic fractures, which would contribute to oil storage and flow. Mineral interparticle, mineral intraparticle and organic matter pores are well developed in the rocks studied. Mineral dissolution has created secondary pores in some areas, such that reservoir capacity is increased. The increased concentration of organic acids between 2800 and 3500m is likely to lead to enhanced shale oil prospectivity. The micro-fractures and dissolution pores are probably the main reservoirs for shale oil accumulation. The association of these features with saline facies potentially offers an exploration tool for further shale oil occurrences in the studied area, as well as in other areas containing similar lithologies.