The Role of Seismic Data in the Exploration and Development of Carbonate Karst Cave Reservoir
Authors: Cai Zhenzhong , Zhao Chunduan , Deng Xingliang , Tong Yanming , Pan Yangyong , Yang Pin , Lu Guanghui 
Published: May 2016

In the Tarim basin of northwest China, large karst caves filled with oil and water have been found in Ordovician carbonate formation at depths exceeding 6,000 m. Sudden "bit drops" exceeding 10 m are not uncommon during drilling into such formation in the area, making logging data difficult to acquire. It is believed that wells drilled in the basin did not directly encounter with the caves but experienced high production are because of nearby caves. The well log data does not represent the productive reservoir. The relationship between reservoir properties and elastic parameters cannot be acquired from rock physics, reducing effectiveness of prestack inversion.

The seismic reflection of karst caves is related to their size, shape, and content as well as the seismic signal to noisy ratio. Reservoir characterization using seismic techniques is a challenge.

An innovative, integrated approach of four steps was taken to overcome this challenge. First, seismic reservoir interpretation from four levels of the target zone was performed based on the knowledge from drilling and the seismic data reprocessed with high fidelity and preserved amplitude. Then, seismic attributes were extracted along the interpreted horizons and the statistics of seismic attributes, which can represent the caves, were measured and evaluated. Next, well production was normalized for statistics on more than 20 production indicators (such as one-month, 6-month, and one-year oil equivalent). Finally, cross plots were made to find the relationship between seismic attributes with production indicators. Based on the results, fifty drilling targets were proposed and accepted by an operator. The integrated approach was verified based on drilling results.

Introduction

The study area is located in the west of the TaZhong area, Tarim basin, in China. Ordovician Karst reservoir is the target for oil exploration and production. Well-Driven-Seismic data reprocessing has improved the seismic images on caves and cave-fracture karst features. The seismic data used for the project has relative high fidelity. Prestack invertion was done using statistics wavelet as the sonic logs were not longer enough.

The type of carbonate karst reservoirs is abundant, they consist mainly of tectonic fractures, diagenetic fractures, styloite, matrix porosity, dissolution porosity, and karst caves. Among them, the highly heterogeneous karst caves are the main resvervoir space for hydrocarbon accumulation. In seismic section, karst caves show as "a string of beads" (Figure 1), which is a vertical stack of positive amplitudes and negative amplitudes. From the seismic section we can see the length and magnitude of the strings of beads is different. This may be related to the cave size and shape. During the past few years, many studies including forward modeling have been carried out in Tarim basin. The relationship between the seismic attibutes and cave size, cave shape and content inside is complicated.