Impact of Thermal Maturation of the Upper Cretaceous Bituminous Limestone of Attarat Um Ghudran Central Jordan on Calcareous Nannofossil Preservation

Abstract

Oil shale deposits of the Late Cretaceous from three boreholes in central Jordan were examined to assess the impact of thermal maturation on the content of nannofossils. Thermal activity has been shown to have a strong effect on organic matter content and composition but its effect on calcareous nannofossil assemblages remains inconclusive. This study aims to determine the impact of thermal maturation on nannofossil assemblages and to compare this to an estimated maturity level based on bulk geochemical analysis. Micropaleontological and geochemical analyses were conducted on 31 samples from three oil shale wells drilled in Attarat Um Ghudran central Jordan. Several types of nannofossil preservation have been recorded, including dissolution, overgrowth, and breakage. In the Jordan oil shale sections, nannofossils exhibit a variety of preservation types, with intense dissolution in the middle part of the study sections. The vast majority of the samples had high TOC enrichment, with 29 samples exceeding values of >10%. Kerogen recovery and quality from the oil shale are very good, with a predominance of fluorescent amorphous organic matter (AOM) and minor algal components. The low fluorescence preservation index (FPI), which is 1 in most of the samples, indicates that alteration occurred due to intense thermal activities in the study interval. The palynomorph and AOM fluorescence, ranging from a spore coloration index (SCI) of 3 to 5, suggest that the studied samples were approaching the oil window. A correlation between the nannofossil preservation and geochemical parameters shows a predominance of poorly preserved nannofossils along with high total organic carbon contents and an elevated hydrogen index (HI). We show that low FPI values and a higher level of maturity are associated with poor nannofossil preservation, suggesting that nannofossils, in conjunction with petrographic analysis of kerogen, could be used as a rapid screening technique for estimating levels of oil-shale maturity. The nature of the tectonism in the study area, including faulting and a metamorphosed zone, enhanced the maturity, which might explain why the nannofossils were so significantly affected.

Geochemical and Organic Petrological Characteristics of the Bituminous Carbonate Succession (Upper Cretaceous Shu’ayb Formation) in Northern Jordan: Implications for Organic Matter Input and Paleosalinity, Paleoredox, and Paleoclimatic Conditions

“…The terrigenous material originated from eroded bedrocks around the basin and was transported by freshwater input or wind flow . In this case, fresh water flows toward the basin in the humid seasons pouncing stratification of water column in an estuarine system, where saline water trapped below the fresh water due to the existence of highland areas (swells). , In such situation, nutrients could be supplied from the continent through hydrologic cycles in humid and warm periods, and thereby the high marine primary productivity and enhanced riverine runoff in the basin likely induced enhanced water column stratification conditions (Figure ). Those trapped water bodies were characterized by low oxygen content (Figure ).…”

Calcareous Nannofossils Biostratigraphy of Late Cretaceous–Paleocene Successions from Northern Jordan and Their Implications for Basin Analysis

“…Locally, the Cretaceous-Paleocene periods show a rise in sea level, which is reflected in the rock types by which the deposition of the MCM. These rises were coupled with variations in the elevation and sedimentary influx enhancing the preservations in several sites along Jordan and neighboring countries [47][48][49].…”

“…In particular, Arkhangelskiella cymbiformis, Watznaueria barnesiae, Meculla decussata, Zeugrhabdotus spp., Nephrolithus frequens, Gartnerago segmentatum, and Kamptnerius magnificus typically serve as indicators of lower photic zones, signifying their presence in cold marine environments [1,15,29,40,50,51]. On the other hand, the Low Nutrient Index (LNI), based on preferences for species with low nutrient requirements, designates P. cretacea, E. turriseiffelii, W. barnesiae, and M. decussata as low fertility species [15,49].…”