Integration Success Story in Shilaif Shale Oil from Vertical Pilot to Horizontal

The development of unconventional target in the Shilaif formation is in line with the Unconventional objective towards adding to ADNOC reserves. For future optimization of development plans, it is of utmost importance to understand and test and therefore prove the productivity of the future Unconventional Horizontal Oil wells. The Shilaif formation was deposited in a deeper water intrashelf basin with thicknesses varying from 600 to 800 ft from deep basin to slope respectively. The formation is subdivided into 3 main composite sequences each with separate source and clean tight carbonates. The well under consideration (Well A-V for the vertical pilot and Well A-H for the horizontal wellbore) was drilled on purpose in a deep synclinal area to access the best possible oil generation and maturity in these shale Oil plays. Due to the stacked nature of these thick high-quality reservoirs, a pilot well is drilled to perform reservoir characterization and test hydrocarbon type and potential from each bench. Fracturing and testing are performed in each reservoir layer for the primary purpose to evaluate and collect key fracturing and reservoir parameter required to calibrate petrophysical and geomechanical model, landing target optimization and ultimately for the design of the development plan of this stacked play. Frac height, reservoir fluid composition and deliverability, pore pressure are among key data collected. The landing point selected based on the comprehensive unconventional core analysis integrated with petrophysical and geomechanical outcomes using post vertical frac and test results. Well A-H was drilled as a sidetrack from the pilot hole Well A-V. This lateral section was logged with LWD Triple Combo while Resistivity Image was acquired on WL. Based on the logging data the well stayed in the target Layer / formation, cutting analysis data for XRD and TOC was integrated with the petrophysical results in A-H well. Production test results from subject were among the highest rate seen during exploration and appraisal of this unconventional oil plays and compete with the current commercial top tier analog unconventional oil plays. Achieving those results in such early exploration phases is huge milestone for ADNOC unconventional exploration journey in UAE and sign of promising future development.

Characteristic of Tuwayil Formation and New Insight into its Contribution in Middle Cretaceous Petroleum System, Western UAE

In Western UAE, the Middle Cretaceous petroleum system is composed of Shilaif source, Mishrif/Tuwayil reservoir and Tuwayil/Ruwaydha seal. Oil is discovered in Tuwayil sandstone in DH and NN fields. Well correlation of Tuwayil siliciclastic interval shows high heterogeneity and rapid lithology varies. Currently, a few general studies about Tuwayil sandstone was published. However, detailed sedimentary facies, reservoir characteristics and accumulation mechanism about Tuwayil are ambiguous. Limitation on these aspects prohibits enlarging exploration activity of Tuwayil and makes barriers to deepen understanding of the whole K2 PS. To enhance understanding on Tuwayil formation, well data in DH, NN fields and adjacent area was integrated. Dedicated single well analysis, well correlation and petrophysics study were carried out. Cores were observed and laboratory outcomes including TS, SEM, RCA, MICP, XRD were adopted into this study. Furthermore, we have also utilized 2D&3D seismic to illustrate the spatial distribution of Tuwayil siliciclastic setting and interior sediment pattern. Basically, the Tuwayil sand-shale interval represents the infilling of Mishrif/Shilaif intrashelf basin and mainly deposits in the tidal flat-delta facies. The epi-continental clast is sourced from the Arabian shield and transferred from west to east. In Western UAE, the Tuwayil depocenter located in DH field, where 4-5 sand layers deposit with net pay of 30-40ft. In NN field, only one sand layer develops with net pay about 4-6ft. Through deposition cycles identification and seismic reflection observation, two sand groups could be recognized in this interval. The lower group is constrained in the depocenter and influenced by the paleo-geomorphology background. The upper group overpassed the former set and pinched out around north of NN. The Mishrif/Shilaif slope area is another potential belt to enlarge Tuwayil discovery, where stratigraphic onlap could be observed and it probably represents the sand pinch-out in lower sand group. For the K2 PS, previous study believed the shale between Tuwayil sand and Mishrif separate these two reservoirs and works as cap rock for Mishrif grainstone. This study suggests that this shale is too thin and not continuous enough to hold the hydrocarbon in Mishrif. On that note, Tuwayil sand and Mishrif belong to the same petroleum system in NN and may have the same OWC. In the NN field, it is quite crucial to consider the extension of Tuwayil sand during evaluating the stratigraphic prospect of Mishrif because the hydrocarbon is mostly likely charged Tuwayil sand first and then gets into underlain Mishrif. This study provides updates and understandings on sedimentary facies, depositional pattern, hydrocarbon accumulation mechanism, reservoir extension and potential identification of Tuwayil formation, which has inspiring implications for the whole K2 PS and could also de-risk the further exploration activity in Western UAE.

Chapter 7 Exploration implications

Exploration of the Jurassic hydrocarbon system in the Arabian Intrashelf Basin area is in a mature state. Given the scale of the present day anticlinal structures and the adjacent synclines, all of the supergiant conventional fields trapped in huge anticlines have already been discovered. The theme throughout this Memoir has been to present the evolution of the self-contained Callovian–Tithonian Arabian Intrashelf Basin hydrocarbon system. Its size, c. 1200 × 450 km, is greater than that of the UK, larger than the Black Sea and almost as large as Turkey or the area of Texas and New Mexico in the USA. It is geologically much simpler than these regions, both in the exceptionally remarkable continuity of facies within the sequences that developed and filled the intrashelf basin and its relative tectonic simplicity, including up to the present day. The cross-sections, facies maps, depositional profiles and other data and interpretations presented in this Memoir have documented this remarkable continuity. The source rock interval is well-defined everywhere it occurs and is mature; enough oil has been generated and migrated so that every sealed trap with reservoir facies will have oil. Around and within the basin, shallow water ramp facies in each sequence are in the reservoir facies and the early-formed porosity has been preserved. The carbonate seals and, even more so, the evaporite seals are remarkably laterally continuous. Therefore the big issue in future exploration is finding a sealed trap with potential reserves large enough to be worth drilling when compared to known reserves and estimates of future production. This chapter discusses some possibilities for stratigraphic traps and unconventional plays. Potential plays have been and/or can be identified, but finding them in the present day structural setting is likely to be very difficult.

Chapter 1 Introduction to the Jurassic Arabian Intrashelf Basin

The Jurassic Arabian Intrashelf Basin provides the setting for the world's greatest conventional oil reserves, including the world's largest oilfield, the supergiant Ghawar field. The stratigraphic interval corresponding to the development and infill of the Arabian Intrashelf Basin is from the uppermost Dhruma Formation to the top of the Hith Anhydrite Formation, spanning the late Bathonian–early Callovian to Tithonian. Many areas of the intrashelf basin have been well described in recent years and the stratigraphic succession has been defined in sequence concepts, but the regional development of the intrashelf basin has not been well synthesized. This Memoir builds on published data to give a regional interpretation of the geological evolution of the Arabian Intrashelf Basin. This introductory chapter reviews some of the earlier work, summarizes the key events and elements in the geological history of the Arabian Intrashelf Basin and gives a brief review of the history of petroleum exploration in this region. It is intended to serve as an extended abstract to introduce the general setting and summarize the contents of this Memoir, including some of the proposed revisions of depositional models, correlations and the sequence nomenclature, providing a context for considering and evaluating each subsequent chapter. The themes summarized in this chapter are documented and discussed in much greater detail in the subsequent chapters of this Memoir.

Chapter 8 Summary and conclusions

This chapter summarizes the main conclusions in this Memoir. Included is a comparison of the Arabian Intrashelf Basin events to the Haq (2017) revision of the Jurassic sea level curve. One conclusion is that the Mid- to Late Jurassic Arabian Intrashelf Basin, with its stable tectonic setting during deposition, extensive present-day outcrops and subsurface well and seismic control, may be an ideal place to evaluate global changes during its time span. The last part of this chapter lists and discusses topics and questions raised within this Memoir which warrant further evaluation.

Chapter 6 Jubaila–Arab–Hith sequences

During deposition of the Jubaila-Arab-Hith sequences, the most important Arab-D reservoir, the Arab-C to A reservoirs and the Arab and Hith anhydrite seals were formed. The Kimmeridgian Jubaila-Arab-D transgression (MFS J70) restored and expanded the intrashelf basin geometry. The restored basin was filled by deeper subtidal Jubaila facies, which graded upwards to the Arab-D reservoir facies. In the late stages the Arab-D facies included both basin rim shoal facies and at least one large isolated shoal surrounded by ‘lagoonal’ intrashelf basin remnants interconnected by accessway channels. This differentiation was facilitated by local structural growth of a few metres. The Arab-D reservoir is overlain by the Arab-D anhydrite, deposited initially as gypsum in broad salinas, which blanketed the intrashelf basin region, forming the first major seal. By the end of Arab-D anhydrite deposition, the entire intrashelf basin area had been filled and transformed into a broad evaporite platform. Atop this platform, the Arab-C to -A (MFS J80-100) alternating shallow water carbonate and anhydrite (initially gypsum) intervals were deposited, ending regionally with the thick Hith Anhydrite Formation. The depositional setting for each of the main Jubaila-Arab-Hith sequences is reviewed and illustrated. The evolution of Jubaila-Arab-D reservoir models is discussed. Different interpretations of Jubaila-Arab-D-Hith sequences are compared, including the issue of whether the Arab carbonate-evaporite depositional sequence boundaries should be taken at the top of the carbonate intervals or the top of the anhydrites. Examples are included which illustrate how easterly palaeowind directions were an important factor controlling Arab-D facies variation. A scenario is presented to show how the Late Jurassic Tethys shelf uplift and low angle unconformity coupled with westward structural tilt were important. An alternating balance developed between the evolving land barrier and eustatic sea-level change during deposition of the Arab carbonate–evaporites and the Hith Anhydrite. Carbonates were deposited when the barrier was inundated and evaporites when the barrier was exposed and the area restricted and hypersaline. A major late Tithonian flooding event (MFS J110) formed the top-Hith Manifa carbonate reservoir and brought open marine carbonate mud-rich deposition to the area, continuing without hiatus into the Early Cretaceous.

About this title - The Middle and Late Jurassic Intrashelf Basin of the Eastern Arabian Peninsula

This Memoir provides a thorough regional synthesis of the geology of the rimmed Arabian Intrashelf Basin, reconciling differing interpretations of lithostratigraphy, sequence stratigraphy and biostratigraphy. Variation of energy levels and facies due to its setting in the SE palaeotradewind belt are described. The intrashelf basin formed during rising sea level as a single rimmed carbonate intrashelf basin. A possible global cooling phase resulted in a lowstand which restricted the basin, resulting in petrographically unique carbonate source rock facies dominated by cyanobacterial deposition. Two subsequent third-order carbonate sequences largely filled the basin. Eustatic change concomitant with uplift of the Tethys shelf resulted in alternation of carbonates and evaporites (gypsum-anhydrite) across the region. The end result was a sealed intrashelf basin which preserved early formed porosity and confined generated hydrocarbons within the intrashelf basin facies.

Chapter 4 Depositional geometry at selected locations around the basin

This chapter includes 11 cross-sections and one well log profile to show the depositional geometry and setting in specific areas around the basin: the Saudi Arabia outcrop belt; the Rimthan Arch; and the eastern and central areas of the intrashelf basin in Saudi Arabia, Bahrain, Qatar, Abu Dhabi and Oman. These cross-sections are used to demonstrate the similarity and degree of continuity of the upper Dhruma Formation, the Tuwaiq Mountain Formation, the source rock, the Hanifa, Jubaila–Arab and Arab–Hith formations and depositional sequences in these different locations in the basin. They show the manner in which the underlying platform formed, the rim developed, the source rock was deposited and the basin progressively filled. The blanket deposition of the Arab-D anhydrite was followed by the Arab-C to Arab-A and Hith carbonate and evaporite sequences. The cross-sections provide the framework used in subsequent chapters to make a series of facies maps and other interpretative diagrams and cross-sections that summarize and, for some intervals, revise the interpretation of the settings and geological events that formed the Arabian Intrashelf Basin.

Chapter 3 Lithostratigraphy and depositional characteristics, age dating and sequence stratigraphy

This chapter reviews the lithostratigraphy, biostratigraphy, age dating and sequence stratigraphy of the Arabian Intrashelf Basin to document the basic data used in the interpretations in this Memoir. The factors important to understanding these topics are discussed, including the typical facies, general depositional models, the effects of dolomitization, well log characteristics and the problems with obtaining precise age dates. Comments are included to show the interpretations preferred in this Memoir.