Unconventional petroleum in fine grained rocks
Creators: name: Mustafa, Karwan
Publisher: name: Imperial College London
Year: 2016

description: Unconventional petroleum of fine-grained rocks has become an interesting field of study and respectively source rocks turned to excellent reservoirs for petroleum generation and accumulation. Exploration and production from fine-grained rocks, such as shale, contemporaneous with technology advancement facilitated black shale use and made them cost benefit. This thesis deals with estimation of unconventional petroleum potential of Paleozoic fine-grained rocks in Poland and investigation of Purbeck rocks in southern England as natural laboratory analogues for unconventional petroleum characterisation. Geochemical results of black shale and limestone samples from the Holy Cross Mountains (HCM) in Poland were used as a proxy to estimate the unconventional petroleum potential of Paleozoic rock successions. Additionally, investigations of oil impregnated shales and limestones in Dorset provided analogues for understanding unconventional petroleum behaviour in tight limestone reservoirs. The rock samples were analysed using total organic carbon content (TOC), Rock-Eval pyrolysis, Gas Chromatography-Mass Spectrometry (GC-MS), Pyrolysis-GC-MS, and X-ray diffraction. Geochemical analyses indicate that the organic matter from the Polish Paleozoic samples in the HCM area were sourced mainly from marine sources and partially from terrestrial inputs that are poorly preserved and possess little potential for hydrocarbon generation. Devonian and some Silurian rocks, however, have good generation potential and can be considered as potential source rocks. The depositional conditions were mixed between oxic and anoxic and settings varied from shallow marine to deep marine along Baltica. There is a substantial change in organic contents and depositional environments across the Ordovician-Silurian boundary due to Hirnantian glacial events and subsequent deglaciation and transgressions. Organic matter increases across the boundary as a result of organic productivity caused by an upwelling system and marine transgressions subsequently changed the shallow marine environments to deeper water settings. The organic matter of the Paleozoic successions in the HCM is composed of type II or mixed type II/III kerogen that has a variable maturity ranging from marginal maturity to peak mature levels. The mineralogical composition of the studied shale and limestone samples in the HCM comprises quartz, carbonates, and clay minerals. Cambrian samples are generally poor in quartz and contain high amounts of clay minerals. Ordovician, Silurian, and Carboniferous shales contain high amounts of brittle minerals convenient for artificial stimulation although a few samples contain about 40 % clay minerals. The Devonian, one Carboniferous sample, and Permian samples are composed of carbonate minerals (calcite and dolomite) with small amounts of quartz. In terms of unconventional petroleum, the analysed rock samples in the outcrops and quarries cannot be considered as potential targets because TOC, maturity, mineralogy, or shale thickness would not fulfil the minimum requirement. However, the Devonian black shales could be regarded as oil shales but have limited potential due to thin nature of the beds. Existence of more interested fine-grained rocks in terms of unconventional investigations might be possible in areas, outside of the HCM, towards Carpathian Foredeep and NE of the HCM owing to the structural setting of the area. Non-impregnated black shales and limestones analysed from the Purbeck Group across Dorset, southern England indicate that there are lateral variations in distribution of organic compounds and depositional conditions. Marine, freshwater, and terrestrial organic matter was found in the samples and higher amounts of organic matter was preserved in central parts of the studied area where anoxic conditions and water column stratification were prevalent. The organic matter did not enter the oil window and exhibit immaturity or marginal maturity. Although freshwater was flowing from the west, paleosalinity was high in the west and decreased towards the east where marine incursions changed the water salinity. Freshwater and terrestrial organic materials were contributed from north and west of the studied area by the rivers and seasonal run offs from the hinterlands as a result of sea level fluctuations, local tectonic uplift, and climate changes. Some of the black shales and limestones of the Purbeck Group in Dorset were impregnated with oil and exhibit variations in oil chemistry. High amounts of oil staining were recorded in the shales with partial impregnation in the limestones, mostly along their contacts with the shales. Mixed sources of organic matter was found in the samples relating to autochthonous lacustrine (fresh-brackish water) organic materials indigenous to the Purbeck rocks and marine organic matter from migrated hydrocarbons. Two different maturity levels were found in the samples, immaturity for the autochthonous organic matter and peak oil window maturity for migrated hydrocarbons sourced from Lias rocks. Molecular characterisation revealed that there are different biodegradation levels between the shales and limestones, with moderate to heavy biodegradation levels in the shales and light biodegradation levels in the limestones, in particular for the limestones above the shale bed. Biodegradation levels increase westward across the area and at greater distances within the limestones surrounding the shales. Four different types of hosts for oil impregnation in the limestones were documented which include pores, fractures, matrix, and gastropod shells. Oil chemistry and its characteristics are different between hosts of varying sizes and volumes. Oil staining was highest in large pore spaces and large gastropod shells and was dominated by polar constituents; the limestone matrix was the least stained host in which aliphatic fractions prevailed. The oil in the hosts was biodegraded to different levels, moderate to heavy degrees in large pore spaces and fractures and light level in the matrix and small gastropod shells providing a composition more or less similar to the original composition of the migrated oil.