Evolutions of Oil Generation and Expulsion of Marine-Terrestrial Transitional Shales: Implications From a Pyrolysis Experiment on Water-Saturated Shale Plunger Samples

Shale reservoirs are characterized by self-generation and self-accumulation, and the oil generation and expulsion evolution model of organic-rich shales is one of important factors that obviously influence the enrichment and accumulation of shale oil and gas resources. At present, however, relevant studies on marine-terrestrial transitional shales are inadequate. In this study, a pyrolysis experiment was performed on water-saturated marine-terrestrial transitional shale plunger samples with type Ⅱb kerogen to simulate the evolutions of oil generation and expulsion. The results indicate that marine-terrestrial transitional shales have wider maturity ranges of oil generation and expulsion than marine and lacustrine shales, and the main stages of oil expulsion are later than those of oil generation, with corresponding Ro values of 0.85%–1.15% and 0.70%–0.95%, respectively. Although the oil generation and expulsion process induced a fractionation in compositions between the expelled and retained oils, both the expelled and retained oils of marine-terrestrial transitional shales are dominated by heavy compositions (resins and asphaltenes), which significantly differs from those of marine and lacustrine shales. The kerogen of marine-terrestrial transitional shales initially depolymerized to transitional asphaltenes, which further cracked into hydrocarbons, and the weak swelling effects of the kerogen promoted oil expulsions. The oil generation and expulsion evolutions of these shales are largely determined by their organic sources of terrigenous higher organisms. This study provides a preliminary theoretical basis to reveal the enrichment mechanism of marine-terrestrial transitional shale oil and gas resources.

Timing of oil expulsion from source rocks and a revitalization of the pre-1970 model

New data from North Sea Upper Jurassic source rock samples show no decline in the total amount of organic matter (TOC) within the oil expulsion window between 120 and 150°C which is a key prediction by today’s model for oil expulsion. However, today’s model for oil expulsion is not consistent with either subsurface source rock TOC data or chemical attributes of shallow oils. Instead, these data are more consistent with oil expulsion occurring at much lower temperatures and shallower depths, more similar to models advocated by most oil explorers prior to 1970 where the oil was assumed to have expelled at burial depths less than ~2km. In this paper, main oil expulsion has been determined to be take place at burial depths less than 1km and approximately 30°C. The oil is mobilized by CO2 gas which is generated from decomposing organic matter and is predicted to migrate out of the source rock and into nearby high-permeable rocks via horizontal fractures that originate from loadbearing swelling organic lamina and in a direction towards decreasing overburden. The thermally immature (heavy) oil is then converted to light crude within the reservoir oil starting at 60-70°C by hydrogenation. Hydrogen gas is common in subsurface fluids and is provided to pooled oil from coalification of organic matter in mudstones. Thus, if the supply of hydrogen is limited, in-reservoir thermal upgrading will be hampered. In this model, most of the heavy oil accumulations encountered are immature rather than due to biodegradation of mature oil at low temperatures.

Petrophysical properties of bypassed Cenozoic clastic reservoirs in the Cesar sub-basin, Colombia

The Cesar-Ranchería basin has all the necessary elements for the generation, expulsion, and migration of hydrocarbons and considerable potential for coal bed methane (CBM) in Colombia. Previous studies in the Cesar basin focused on understanding the tectonic evolution, stratigraphy, hydrocarbon generation potential, and evaluation of reservoir potential in Cretaceous calcareous units and quartzose sandstones from the Paleocene Barco Formation. These studies had confirmed the existence of an effective petroleum system, with several episodes of oil expulsion and re-emigration in the Miocene period, turning the Cenozoic clastic succession (Barco, Los Cuervos, La Loma, and Cuesta formations) into an element of significant exploratory interest to clarify the potentiality of the basin in terms of hydrocarbon accumulation. The petrophysical parameters of Cenozoic units (shale volume, porosity, water, and oil saturation) were determined by integrating wells log and core samples analyses from three stratigraphic wells. The integration of these results synthesizes the petrophysical behavior of the units. It defines intervals with clay volumes of less than 30%, effective porosity around 20%, which means favorable characteristics as reservoir rocks that need to be considered in future exploratory projects.

Geochemistry Evaluation and Oil to Source Rock Correlation of Lhokseumawe Area, North Sumatra Basin

The purpose of this study was to evaluate the hydrocarbon source potential of the Middle Miocene Lower Baong Formation of the Lhokseumawe area and further attempt to establish a correlation of this possible source with the oil produced in the area by the mean of biomarkers analysis. Source rock characterization was realized by integrating several geochemical measurements including TOC, HI and Ro from the DY-1 and TM-1 wells. This has allowed us to define the organic content, maturity and kerogen type of the Lower Baong. Meanwhile, multiple produced oil samples from the area of interest were used to characterize their geochemical signature, based on a combination of isoprenoid, triterpane and sterane biomarkers, in an effort to determine maturity of those oils and the depositional environment of their source. This provides the basis of our attempt to correlate genetically oil and source rock in the Lhokseumawe area. The geochemical characterization of the Lower Baong suggest the formation to be an effective active source rock in the Lhokseumawe area with remaining average TOC of c1.5%, Kerogen Type II-III and maturity levels optimal for oil expulsion. Biomarker analysis of oils suggest an origin from a source facies deposited in open marine environment. Maturity level analysis using pentacyclic triterpane supports the source to be at peak maturity level. The correlation of oil with source rock supports largely that the Lower Baong oils are sourced by Lower Baong source facies of marine origin. However it worth noting that one oil family differs from this genetic correlation, opening speculation around other source facies to be active in the Lower Baong or an additional source interval to be active in the Lhokseumawe area. This could be interesting topic to be discuss in future.