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.

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.

There are a limited number of studies and exploration cases for a "reworked carbonate" in Abu Dhabi, although these sediments are composed from some large oil and gas fields around the world (e.g. Poza Rica oil field in Mexico and Ruby gas field in Indonesia). In this study, we focused on Cenomanian-Turonian carbonates and considered the depositional processes of a "reworked carbonate" in the eastern part of Abu Dhabi. To understand the stacking pattern and/or depositional process of the Cenomanian-Turonian carbonate, we conducted a well-well correlation for total 16 wells, based on the core observations, wireline logs correlation (GR, Neutron, Density, Resistivity and Sonic), carbon and oxygen isotope analysis and trace elements analysis. Sampling was conducted for 8 wells and samples were taken approximately every 5 ft. In addition, to predict the spatiotemporal expansion of the reworked deposit, a 3D seismic interpretation was conducted. The result of the well-well correlation reveals that the depositional process and the stacking pattern of the Cenomanian-Turonian shoals around eastern Abu Dhabi are well consistent with the depositional model that proposed by Razin et al., 2010, and the reworked deposits are developed around the distal environment. 3D seismic interpretation represents that these reworked sediments were input from the north-west side and spread to the south-east like as a submarine-fan. Considering the core observation result, cohesive debris flow deposits are dominated at the depositional up-dip side and dilute flow deposits are dominated in the depositional down-dip side. In addition, an obvious erosional surface can be recognized in seismic sections and it truncates the top shoal sediments. The result of both, a combination of localized up-rift and global eustatic sea level fall in the early-middle Turonian triggered the regional erosion which is recognized as the middle Turonian unconformity. The result of this study suggests that the shoal sediments were eroded and reworked to a more distal environment at the early-middle Turonian.

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The central part of the Cenomanian intrashelf basin in Abu Dhabi is composed of mudstones with high TOC concentration. The mudstone with high TOC in the Cenomanian interval is divided into three zones as named of OAE1d, MCE, and OAE2 of OAE (Ocean Anoxic Events). In this study, the two wells (Well A and Well B) were re-evaluated to confirm carbon isotope curve (chemostratigraphy) and geochemical/biomarker analysis including TOC measurement and to consider paleo environment developing these mudstones within the intrashelf basin. Regarding this intershelf basin, there are several papers to study of the relationship between TOC (Total Organic carbon) and OAE (Ocean Anoxic event) using geochemical analysis. We conducted core observations, GR log correlation, carbon and oxygen isotope analysis, trace elements analysis, mineral mapping, TOC analysis and biomarker analysis of two wells. Sampling was taken at 3ft for both wells. The results of carbon isotope were almost the same as the OAE zone (three OAE zone from lower to upper OAE1d, MCE and OAE2) within previous studies. Flat ammonite/inoceramus in OAE1d mudstone were recognized in both wells. It is highlighted that the ammonite/inoceramus indicates a good example to understand the productivity and preservation of the OAE. The cross lamination of mudstone contained ammonite/inoceramus had not received any bioturbation, therefore, the OAE1d interval is considered to have been buried earlier than biodegradation. This mudstone shows the alternation of gray- and dark-colored (each thickness is 10-30cm) and high TOC contents around 11-13%. However, mineral mapping suggested that illite and pyrite were only partially confirmed in well B. Same samples were increasing C35 Homohopane. Regarding paleo topography, well B was located near hinterland area, which provided carbonate debris with clay minerals. In addition, the results of two wells are plotted with variation of C28 sterane in the ternary diagram. However, oil samples in the Afield located on east side of intershelf basin are plotted in the almost same area in the ternary diagram. The differential was dependent on seawater circulation between intrashelf basin and open marine of Eastern Tethys. By combining the newest technologies like carbon Isotope, ICP-MS, mineral mapping analysis and biomarker analysis, this allows an in-depth understanding of regional geological interpretation. In addition, core and thin section observation is an essential part to the geological understanding.