We investigated the method of estimating porosity/permeability using X-ray CT, a non-destructive method. Using X-ray CT, a method of estimating the porosity/permeability is particularly developed in sandstone. However, for the carbonate rocks, the internal structure is complicated due to biological origin. This is difficult to recognize the pore space, therefore a method of estimating the porosity/permeability using X-ray CT has not been studied. This study is based on Yamanaka et al. 2018, which clarifies rudist development in side slab core using X-ray CT and 3D modeling. The study uses X-ray CT to observe the internal structure from the view of development of rudist of the 200 feet section of the Well A in offshore Abu Dhabi, and compares the porosity/permeability obtained from CCA (Conventional core analysis) of the same well and same interval. Based on the 3D modeling of the X-ray CT, two rudist families (Radiolitidae and Ichthyosarcolites) were identified through their morphological characteristics such as inner diameter and shell thickness. A porosity of slab core around 50 feet is about 18% from CCA (Conventional Core Analysis). This slab core is made up of small rudist populations (length and wide size is 15-10mm), inside core confirmed 3D modeling (surface rendering and volume rendering), and calculated porosity is 0.89% from RCM (Reverse Coupling method). It is understood that this difference is dependent on matrix porosity and further investigation in the future is required in order to measure matrix porosity using thin section and micro X-ray CT. With regards to reservoir properties, the porosity is higher in the lower part than the upper part in the core interval. The size of the Radiolitidae could be dependent on the environment and its vertical variation suggests the change of depositional environment. Larger Radiolitidae, which appeared from 80 to 200 feet below the C-T (Cenomanian-Turonian) boundary, suggests a relatively strong wave influence. From a sedimentological point of view, the coarser matrix grain size supports the interpretation of depositional setting. On the other hand, from 30 to 80 feet below C-T boundary, smaller Radiolitidae is dominated. It was assumed that small Radiolitidae could be due to high physical stress under a restricted environment. This study shows the advantage of X-ray CT image in rudist recognition, based on interpretation of depositional environment and understanding the reservoir property. The result of this study suggests the strong correlation between porosity/permeability and depositional environment (accommodation space) inferred from rudist fossil.

This study investigates overpressure evolution and its relationship to the spatial and temporal changes in the sedimentation rate in the passive continental margin of offshore Suriname. We analysed well data to estimate pore pressure at the well locations and to interpret relevant overpressure-generating mechanisms. Three-dimensional basin modelling was performed to reconstruct overpressure evolution in the area. Highlights • Overpressure evolution and its relation to sedimentary history were examined in offshore Suriname. • Temporal changes in sedimentation resulted in cycles of overpressure generation and dissipation. • Spatial variation in sedimentation caused lateral changes in the amount of overpressure. • The sedimentation rate primarily controlled the overpressure evolution in the region. • The overpressure history in offshore Suriname is linked with the hinterland evolution. How to cite this article: Nifuku K, Kobayashi Y, Araki Y, Ashida T, Taniwaki T. Overpressure evolution controlled by spatial and temporal changes in the sedimentation rate: Insights from a basin modelling study in offshore Suriname.

Rudists are a group of strange shaped marine bivalves lived in the Tethys Ocean from the Late Jurassic to the Late Cretaceous. The rudist-bearing carbonates form a lot of oil and gas reservoirs in the Middle East. Therefore, the taxonomy, morphology, paleo-ecology of rudists is important to understand the rudist-bearing carbonate reservoir features for oil exploration and development. However, it is difficult to understand these characters of rudists because we can't collect whole rudist samples from the underground oil and gas reservoirs through core sample. X-ray CT is a useful method to visualize three dimensional rudist images with non-destruction of the core. Hence, X-ray CT has a potential to obtain the information of the taxonomy, morphology and depositional environment of rudists from core information. We conducted the X-ray CT scan to the reservoir formation (Formation A) of the Cenomanian age using core slab samples of Well #A and Well #B in the Abu Dhabi oil field. The some rudist fossils were observed on the cutting surfaces of slab cores in the both wells. However, the three dimensional morphology of rudists were not identified inside of the slab core. On the CT images, some autochthonous rudists were identified and it made the colony in Well #A. This rudist is standing position and suggesting original position of depositional environment from Cestari and Sartorio (1995). We demonstrated to be able to obtain the morphology from the slab cores with non-destruction using X-ray CT scan in this paper. Now we are challenging to make the high resolution 3D image modeling of rudists based on this X-ray CT result. This paper is summarized that usage method of the X-ray CT result to understand taxonomy/morphology and depositional environment from three dimensional position of rudists In addition, in the future technique, this paper suggested that combined technique between X-ray CT of core and FMI may reveal more comprehensive depositional setting such as direction of paleo ocean current and paleo wind in the future.

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Identification of hydrocarbon generating source rocks and evaluation of their potential are essential in the exploration and development of hydrocarbon resources. For an offshore oil field in Abu Dhabi, we conducted geochemical study using crude oil and core samples from Upper Cretaceous Cenomanian carbonate rocks. The study objectives are 1) correlation of crude oil and source rock with biomarker, and 2) evaluation of the source rock potential. The Cenomanian carbonate rocks of the oil field are composed of shallow marine porous limestone and deep marine lime mudstone. This Cenomanian lime mudstone was believed as source rock of the crude oil in the interfingered Cenomanian porous limestone reservoirs. However, the origin of crude oil has been poorly constrained with geochemistry yet. In this study, we carried out geological description and RockEval pyrolysis analysis of core samples to evaluate source rock potential of the lime mudstone. Then, biomarkers such as hopane, sterane and compound specific isotopic ratio of n-alkane were analyzed to correlate the source rock and the crude oil samples with GC/MS, GC/MS/MS and GC/C/IRMS for high resolution biomarker measurements and robust interpretation. As a result, the biomarker fingerprints of the crude oil in porous limestone and the organic material in the lime mudstone show significant similarity. It proves that the crude oil in the porous limestone is migrated from interfingered organic rich Cenomanian lime mudstone. In addition, the lime mudstone shows excellent source rock property (Total Organic Carbon exceeding 4%, Hydrogen Index > 600mg/g TOC) and categorized as Type I/II source rock deposited in marine environment. Furthermore, the biomarkers effectively constrain the maturity of source rock which is difficult to evaluate with Vitrinite Reflectance and RockEVAL analysis. Consequently, the timing of hydrocarbon generation and the area of effective source rock will be interpreted based on our study result with higher confidence. This study deepens understanding of Cenomanian petroleum system in offshore Abu Dhabi. The result suggests the advantage of biomarker application not only in oil-source correlation but also in source rock maturity analysis.