Controls on heterogeneity and mechanical behavior in immature organic-rich carbonates: Insights from an inter-well transect, Al-Lajjun, central Jordan

This study investigates the vertical and lateral variability in geochemical, petrophysical, and mechanical properties of immature, organic-rich carbonate source rocks within the Upper Cretaceous Muwaqqar Chalk Marl Formation (MCM) in the Al-Lajjun Graben, central Jordan, using an inter-well transect approach. The study aims to evaluate the heterogeneity of geochemical, petrophysical, and mechanical parameters along the inter-well transect; identify the key controls governing this variability and reservoir quality; and assess the potential of mineralogical and petrophysical data for predicting rock mechanical properties. An integrated workflow was applied to 67 core plug samples extracted from two cored wells, including lithofacies analysis, bulk geochemical characterization, spectral gamma-ray logging (SGR), ultrasonic velocity measurements, micro-rebound hardness testing (MRT), and scratch testing. Multivariate linear and nonlinear regression models were developed to predict mechanical properties from geochemical and petrophysical data. Six lithofacies and three chemostratigraphic zones were identified, exhibiting systematic variations in mineral composition, porosity, total organic carbon (TOC), and diagenetic overprint. TOC values range from 1.9% to 34.7%, and HI values, spanning 662- 1001 mg HC/gTOC, indicate the presence of highly organic-rich, thermally immature Type I and IIS source rocks. Average estimated porosity is ∼30%, while unconfined compressive strength (UCS) varies from 38 to 165 MPa and hardness from 352 to 710 HLD, reflecting pronounced mechanical heterogeneity across the inter-well transect. Nonlinear regression models consistently outperform linear models in predicting mechanical behavior, yielding higher R² values. The results demonstrate that mineralogy, porosity, TOC, depositional architecture, inter-well spatial variability, and diagenetic processes collectively control reservoir quality and mechanical behavior. This integrated framework improves the prediction of mechanical properties from log-derived and geochemical data and helps identify sweet spots for unconventional resource evaluation. These immature carbonate systems also provide valuable analogs for assessing similar unconventional reservoirs prior to thermal maturation.