Abstract

Geochemical techniques have been applied to a complex reservoir in Yemen in order to better understand its compartmentalization and filling history. Such information is of significant importance for effective reservoir management and determination of the remaining potential of the area.

Reservoir compartmentalization: The studied multi-layered reservoir consists of three substructures separated by either a fault or a saddle. By performing a statistical comparison of high resolution gas chromatographic data from the different reservoir levels in the subsequent substructures we were able to demonstrate that both vertical and lateral flow barriers for hydrocarbons exist. Recognition of a lateral barrier in the upper reservoir level is of specific interest because the two substructures separated by the saddle had the same acquifer.

Filling history: Detailed comparison of specifically the light hydrocarbons revealed that this structure was charged from different source kitchens in two different subbasins. The upper reservoir was subsequently affected by waterwashing. These result have a significant impact for the evaluation of the remaining potential of this area.

Introduction

Applied organic geochemistry in the petroleum industry has been primarily exploration oriented, focusing mainly on the source rock evaluation, oil characterization with oil-source and oil-oil correlations including geochemical modeling of oil and gas generation. More recently, organic geochemistry and specifically the use of gas chromatography, have successfully been applied in solving reservoir-related questions, such as vertical and lateral fluid continuity production allocation and predicting gas, oil, and water intervals. In the present paper we discuss the results of a geochemical study on a field in Yemen using these chromatographic techniques in order to resolve the following issues:

  • Vertical and lateral communication within and between different reservoirs,

  • Post generation alteration processes controlling the present composition of the oils,

  • Filling history of one of the reservoir intervals.

Geological setting

The oil field discussed in the present paper is located in a onshore Jurassic basin in Yemen (Fig. 1). The sedimentary section of interest for the discussion here ranges in age from Late/Middle Jurassic to Lower Cretaceous. The general description of the stratigraphy can be summarized as follows (Fig. 2). The fractured basement is overlain by reservoir sandstones of the Kohlan Formation (Callovian). The limestone sequence of the Shuqra Formation cap these sands. Just above, the Upper Jurassic section consists of the Madbi Formation of Kimmeridgian to Oxfordian age. It is composed of marine marls which are considered as the source rock for the oils recovered in the basin. The Madbi Formation is overlain by the Sarr Formation of Neocomian age which is composed of dolomitic limestones which are occasionally argillaceous. The dolomitic horizons at the top of the Sarr sequence have reservoir characteristics in some wells. Above, the main reservoir horizons are developed in the sands of the Biyad Formation of Barremian to Aptian age. These sands are sealed by alternating mudstones and shales of the Qishn Formation which is of Gargasian age.

The field is divided in three substructures. A fault system separates the substructure 1 on which well A was drilled from the substructure 2 perforated by the wells B and D. A saddle separates the substructure 2 from the substructure 3 on which the well C is located. Oil samples recovered from drill stem tests (DST) of the three main reservoir horizons in the four wells were used for detailed characterization. A schematic cross section between the wells A, B, D and C with the depth intervals of each oil test is shown in Fig. 1. P. 281^

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