Identification and Quantification of Parasequences Using Expectation Maximization Filter: Defining Well Log Attributes for Reservoir Characterization

Sinha, Saurabh (University of Oklahoma) | Kiran, Raj (University of Oklahoma) | Tellez, Javier (University of Oklahoma) | Marfurt, Kurt (University of Oklahoma)

OnePetro 

Abstract

Identification and quantification of parasequences remains a key aspect in unconventional reservoir development [1] demonstrated the importance of gamma ray parasequences (GRP) in unconventional play development. Currently, most of the drilling plans in unconventional plays are executed using a ‘factory made” drilling and completion program. Due to thousands of wells in an unconventional play, it is a very difficult task for operators to incorporate the fine scale reservoir characterization in time for drilling plan.

Currently the upward dirtying and upward cleaning parasequences in shale plays are interpreted qualitatively and manually by a human interpreter on individual well logs. We believe these parasequences hold key information about the underlying geology and their quantification can provide key insights into the depositional environment and hence reservoir quality. Incorporating this information in due time for drilling and completion can aid the decision making process on well placement and hydraulic fracturing design.

In this work, we handle the reservoir characterization challenge on two fronts: we first provide a statistical filtering approach to interpret the parasequences in a well log and then use machine assisted application on other wells in the area of interest. We then use Least-squares fit to obtain slopes of these parasequences. Furthermore, we map these slopes and compare them to the conventional parasequence thickness map to provide quantitative well log attributes to help aid the geologic interpretation.

Introduction

Unconventional play development has key differences to that of a conventional play development. In a conventional porosity, permeability etc. are the key drivers for production. Well spacing and landing the best zone and hydraulic fracturing guide the production performance in horizontal wells. As the well is completed with hydraulic fracturing operation, the geomechanical properties of the layer become of utmost importance ([2]-6]). [1] proposed that the layered properties of the shale reservoir are highly complex and is composed of alternating brittle and ductile geological sequences also known as brittle-ductile couplets [7]. The optimal landing zone depends on a tradeoff between the brittleness and rock properties such as total organic carbon(TOC). The good rock from the reservoir perspective which is high in TOC is generally more ductile and not a suitable candidate for hydraulic fracturing operation and vice versa.