Exploration Insights September 2019 | Page 16

Source rock volume Extent of petroleum generation process Volume of generated hydrocarbons Volume of in-place hydrocarbons Calculated from lateral and vertical extent of source rock Calculated from the remaining versus original source rock generative potential Calculated from source rock volume and extent of petroleum generation process Calculated from application of generation accumulation efficiency range Exploration Insights | 17 16 | Halliburton Landmark Figure 1 > The principal workflow steps and outputs to calculate the volume of hydrocarbons generated, and an estimation of the in-place volumes accumulated in reservoirs. A Geochemical Approach to Forecasting the Volume of Preserved Hydrocarbons By: Rachel Round Well pads along the Little Missouri River. Photo © Chris Boyer, Kestrel Aerial Services, Inc. An ongoing challenge in exploration is the assessment of the hydrocarbon resources in a basin. A first step in this process is the calculation of the volumes of hydrocarbons generated. This article presents a workflow that uses organic geochemical and geological data to estimate the volume of hydrocarbons generated without the need to build a complex 3D basin model. A secondary aim is to estimate the amount of in-place petroleum likely to be accumulated in reservoirs. This provides insights that reduce the charge risk present at all levels of exploration maturity, allowing basins to be ranked in terms of their potential. The methodology is illustrated by a case study of the Bakken Petroleum System, Williston Basin. Neftex ® Insights portfolio, principally the Neftex Organic Geochemistry Database, to generate a suite of maps that inform on the source rocks within a petroleum system on a basin scale. These maps are derived from widely available, or inferable, data in agreement with the mindset of Schmoker (1994), who highlighted that calculations such as this should be applicable to a diverse set of petroleum systems, with variable data availability. The individual source rock maps created are: » » Preservation extent » » Isopach » » Organic matter variation via kerogen type ESTIMATING THE VOLUME OF HYDROCARBONS » » Present-day total organic carbon (TOC) Workflow Overview » » Original TOC A workflow (Figure 1) suitable for non-specialists has been developed that pulls together public domain data and understanding held across the » » Original HI » » Present-day hydrogen index (HI) » » Basal maturity The mapped data are combined in a series of simple equations to calculate the source rock volume, the extent of the petroleum generation process, the volume of hydrocarbons generated, and ultimately the amount of in-place petroleum preserved in commercial accumulations in reservoirs. Source Rock Volume Calculating the source rock volume (from lateral and vertical extents) is the first step in the workflow. Source rocks with facies variation may need to be dealt with as more than one unit. The lateral distribution of the source rock is mapped by compiling the organic-rich facies in all relevant Neftex Gross Depositional Environment (GDE) maps, and reducing these to honor observations from data held across the Neftex portfolio; such as source rock occurrence data, depth analysis maps, well and outcrop picks, surface geology, tectonic elements, cross sections, seismic, and isopachs. Additionally, Neftex Depth Analysis frameworks map can be created for the specific source rocks. The vertical distribution of the source rock is presented as an isopach map. This is generated from thickness data, such as well and outcrop data, cross sections, seismic data, isopachs, and data with depths tied to the source rock in the Neftex Organic Geochemistry database. The calculated source rock volume can be integrated with the maturity map to define the volume of mature or post-mature source rock. Extent of Petroleum Generation Process Assessing the extent of the petroleum generation process is critical to estimating the volume of hydrocarbons generated. This is achieved by assessing the amount of kerogen that has been converted to petroleum (kerogen decomposition). The difference between the original (pre-maturation) and remaining (present- day) generative potential of the source rock is calculated in the following steps: 1. Data processing: Geochemical data are compiled, principally from the Neftex Organic Geochemistry database, and screened to determine whether they are applicable. If the data coverage is sparse or patchy, inferences can be made from knowledge of the depositional environment and geological history of the basin. Statistical analysis is carried out on the data as numerous pieces of geochemical data can be recorded at any one location. 2. Remaining source rock generative potential: The variation in the present- day quantity and quality of organic matter in the source rock is assessed by mapping the average present-day TOC and the average present-day HI. This enables any variations across the source rock to be identified that will affect its generative potential. 3. Original source rock generative potential: The variation in the original quantity and quality of organic matter in the source rock is assessed by mapping the average original TOC and the average original HI. Restoration of the original TOC and HI requires present-day TOC, present-day HI, kerogen type, and maturity of the source rock to be known, and the following steps to be followed: a. The relationship between maturity