Genesis and Hierarchy Analysis of Mud-Rich Units Within Braided Fluvial Reservoir: A Case Study From the Orinoco Heavy Oil Belt, Venezuela

Abstract

Despite long time of exploration and production, uncertainty is still significant regarding the subsurface distribution of deposits in a braided river reservoir, Orinoco heavy oil belt, Venezuela. This is because the deposits underneath have complicated internal architectures, which make it challenging to delineate. To solve the problem in an alternative approach, this study focuses on the non-permeable part within the reservoir. Because most of the time, they act as barriers and are potentially more significant for flow behavior and reservoir performance than the permeable part. Therefore, when the distribution and geometries of non-permeable barriers are ascertained, the aim to fully characterize the reservoir becomes much easier and feasible. A detailed study of the lower member of Oficina Formation in H oilfield is carried out first, which includes comprehensive analysis of well logs, core data and petrology description. Then, two categories of deposits are classified: the sand-rich units (e.g. side bars, mid-channel bars, sandy channel fills) and the mud-rich units (e.g. abandoned channel mud plug, channel lag mud, muddy bar drape, lateral accretion mud). In addition, 6 probable geneses about the non-permeable barriers or interlayers are summarized. As for their hierarchies in level, we have considered the hydrodynamic intensity, sediment supply rate, length of time, which are key factors determining the units' scale preserved. Finally, the mud-rich units can be divided into two different categories according to their architectural levels, i.e. developed between and within a single sandbody respectively. In process of the study, techniques such as modern depositional pattern analogy along with empirical formula fitting have been used, to establish the quantitative database of the mud-rich units. We have further revealed key variables such as abandoned channel width, muddy bar drape dimension, in the aid of closely spaced horizontal wells which are very useful especially when high resolution seismic data are unavailable here. The abandoned channels identified are approximately 200m wide, mud-drapes within the bars are 120–300m wide and 540–1200m long. These prediction results are useful in constraining conceptual depositional model and geological model. On the other hand, it is essential for accurately assessing risk as well as developing a successful exploration strategy before drilling new wells.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017