The Unconventional Shale Reservoirs of Jafurah Basin: An Integrated Petrophysical Evaluation Using Cores and Advanced Well Logs

Abstract

The Jurassic mud rocks of Jafurah Basin are one of the most promising shale gas reservoirs in Saudi Arabia, retaining considerably high total organic content (TOC) values, and being the source rock for the world-class oil fields of the Kingdom. The purpose of this study is to build a calibrated model with core data using an integrated formation evaluation approach. The model then is frequently used to estimate reservoir properties, minimize uncertainty, and influence decisions on better well placements. The Tuwaiq Mountain shale play is mainly composed of mudstone with few fraction of dispersed detrital minerals. The Tuwaiq Mountain Formation is divided into two parts: Upper and Lower, where the Lower Tuwaiq Mountain contains higher organic matter and so better reservoir quality as compared to the Upper Tuwaiq Mountain. The formation evaluation of unconventional shale gas reservoirs presents numerous challenges. The conventional porosity logs, density neutron, and sonic, are heavily affected by the presence of organic matter. The estimation of initial hydrocarbon in place requires accurate estimation of formation water saturation. The conventional equations used to estimate the formation water saturation are subject to a high degree of uncertainty, mainly related to a complex wettability system and unknown formation water resistivity. Therefore, these challenges require the use of advanced and calibrated well logs. The advanced well log technologies used in this study are pulsed neutron elemental spectroscopy and nuclear magnetic resonance (NMR). The analysis can only be achieved through comparison and calibration with micro and nanoscale core data to help building an accurate petrophysical model. The use of the pulsed neutron elemental spectroscopy tool allows the estimation of rock composition, the evaluation of the amount of total carbon present in the system, and consequently, the amount of organic matter in the formation. Natural magnetic resonance tools are lithology independent and provide an accurate estimation of total porosity. In unconventional shale gas intervals, the T2 distribution is mainly controlled by the surface relaxation factor, and so can be directly linked to the pore size distribution. By applying an appropriate cutoff, based on SEM results, continuous estimation of organic and inorganic porosity can be directly derived from NMR T2 distribution. A saturation model can also be built from a function, linking formation water saturation and organic carbon porosity. The core analysis data are used to understand the pore structure and calibrate the well logs. The study has proven that the NMR technology works effectively in determining the total porosity, pore system distribution, and estimates of formation water saturation. The nanoscale core analysis is then used to understand the pore structure and to calibrate the well logs.

The Impact of Formation Anisotropy and Stresses on Fractural Geometry—A Case Study in Jafurah’s Tuwaiq Mountain Formation (TMF), Saudi Arabia

“…The liquid-rich Jafurah shale play is located east of the greater Ghawar field, which is the world's largest conventional oil field located in the eastern region of Saudi Arabia (see Figure 1) [23,24]. On the basis of exploration and appraisal programs, the ultimate recovery of natural gas and liquids from the play is estimated to be around 200 trillion cubic ft equivalent (tcfe), and the first commercial production is to start in 2025 [25,26].…”

“…TMF exhibits exceptional unconventional gas characteristics, such as a high total organic content (TOC) and low clay content, and it is in the proper maturity window for oil and gas generation. The thickness of the TMF interval ranges from 110 to 150 ft across the Jafurah basin [23][24][25].…”

“…It is composed of cycles of laminated organic-rich lime-mud wackestones. Figure 2 presents a generalized geological setting of the Middle and Upper Jurassic stratigraphy in Jafurah basin [24,27].…”

Workflow for Probabilistic Resource Estimation: Jafurah Basin Case Study (Saudi Arabia)

“…Thus, the presence of non-swelling clays also makes the formation suitable for hydraulic fracturing. However, the treating pressure needed for wells is higher (around 0.98 psi/ft-1.05 psi/ft) [42]. The significant amount of Although drilling 1000 wells per year may seem a huge number of wells, we recall that, in the combined US shale plays, the drilling rate was ~30,000 wells in 2014, 16,000 in 2015, a dip-due to lasting low oil prices-to 10,000 new wells drilled in 2016, with a steady recovery since to about 21,000 new shale wells drilled in 2021 [38].…”

“…Thus, the presence of non-swelling clays also makes the formation suitable for hydraulic fracturing. However, the treating pressure needed for wells is higher (around 0.98 psi/ft-1.05 psi/ft) [42]. The significant amount of condensate produced during testing of horizontal wells [41] draws attention due to possible risks of condensate banking and possible liquid loading in the wellbores.…”