The Lower Miocene Liuhua Carbonate Reservoir, Pearl River Mouth Basin, Offshore People's Republic of China

Abstract

ABSTRACT Since the drilling in January 1987 of the Liuhua 11-1-lA discovery well, six additional wells, four on the primary structure and one each on two satellite structures, have drilled and tested this lower Miocene reservoir carbonate. Deposition of the carbonates took place in isolated platform environment. Major facies are (1) a platform-rim reef, (2) a backreef 1agoon of fine grained carbonates, (3) a 1arge interior p1atform a1gal bank, (4) platform coarse grained carbonates, and (5) platform to 1agoona1 fine-grained carbonates. A paleo-water table surface represents a time of regional exposure. The reservoir is subdivided into five diagenetic carbonate units. The uppermost unit is a thin, tightly cemented carbonate formed at the time of drowning of the platform. Two thick highly leached carbonate units with porosities and permeabilities as high as 30 percent and several darcies comprise most of the reservoi r. They are separated by a tighter interval which formed by cementation below the water table of the exposure surface. The less porous unit at the base of the reservoir formed as a result of interaction between oil and water caused calcite cementation. Leaching continued in the carbonate below the reservoir and biodegradation occurred after oil had filled the structure. INTRODUCTION Amoco in partnership with Nanhai East Oil Corporation discovered Liuhua Field in January 1987. The field is located offshore China about 220 kilometers (135 miles) southeast of Hong Kong in the Pearl River Mouth Basin of the South China Sea (Figure 1). This field was the first discovery of oil from a 1arge carbonate complex in the basin and is located in Contract Area 29/04 and in water depths over 305 meters (1000 feet).1 The discovery well tested 2240 BOPD of 20°API gravity oil from a lower Miocene carbonate, the Zhujiang carbonate. Since the drilling of the discovery well, four more wells have been drilled on the structure and one each on the satellite structures. The Liuhua Field is a large accumulation and though Amoco has not released recoverable reserve estimates, Nanhai East Oil Corporation, Amoco's partner here, has reported that in place oil exceeds 700 million barrels.2 The Pearl River Mouth Bas in is separated from oceanic crust to the south in the South China Sea by basement highs or massif. The Dongsha Massif is one of these basement highs, and Liuhua Field is located on the western plunge of the Dongsha Massif (Figure 2). Between the basement highs are depressions or sags which developed during the late Cretaceous and early Tertiary. The Dongsha Massif separates two of these depressions. Eocene lacustrine shales (Figure 3) were deposited in the sags or depress ions between the basement highs upon an alluvial section which in turn rests on Mesozoic and Paleozoic metamorphic and igneous rocks. The widespread Eocene lacustrine section is the probable source for the waxy crudes tested by the Chinese and other oil companies to the west and northwest of the Liuhua area. Therefore, the Wenchang is be1ieved to be the major source for oil within the greater Pearl River Mouth Basin.