Summary
A comprehensive geologic and engineering study of the A-10 North pool of theSansinena oil field was conducted to improve recovery of the pool of theSansinena oil field was conducted to improve recovery of the remainingreserves. Two reservoir facies were identified that exhibited distinctivelydifferent reservoir properties and oil production histories. The Channel Coresands have produced 31.7% of original stock-tank oil in place (OSTOIP), arecoarse-grained and locally pebbly, contain very little place (OSTOIP), arecoarse-grained and locally pebbly, contain very little associated clayminerals, and have high permeability and good lateral continuity. The ChannelFlank sands, on the other hand, have produced only 12.3% of OSTOIP, arefine-grained and silty, contain abundant dispersed and laminar clay minerals, and have low permeability and poor lateral continuity. A closer look at theinternal arrangement of reservoir facies, coupled with historical productionperformance, explains the large differences in oil recovery from the ChannelCore and Channel Flank sands and provides a better understanding of theproducing mechanisms within these different facies. The sands with goodpotential for enhanced recovery are identified along with the remaininginfill-drilling and recompletion potentials to enhance primary production fromthis geologically complex reservoir.
Introduction
The Sansinena oil field is located in Los Angeles County along the Whittierfault trend. The oil seepages along the Whittier fault zone, a major geologicfeature in this area, resulted in the discovery of the oldest oil fields insouthern California. The Puente Hills field, discovered in 1880, is the secondoldest oil field in California. This field is located approximately 3 miles [5km] east of the original Sansinena discovery of 1898. Full-scale development ofthe Sansinena field began in 1944, with the major producing zones being theupper Miocene Divs. A, C, and D sands. The Div. A (Zone A-10) reservoirs arethe second most productive in the Sansinena field and are geologically complex, with traps formed by structural and stratigraphic features. Cumulativeproduction from Zone A-10 pools to Nov. 1988 exceeded 13.3 million bbl [2.1 X10(6) m3], a 22.8% recovery of the OSTOIP. Of the 52 wells completed in theA-10 North pool, only seven are currently producing. Only minor gas- andwater-injection projects have been implemented to date, and the pool is in anadvanced state of primary depletion. The original geologic interpretationstreated the entire A-10 North pool as a single reservoir unit with minor faultcomplications. Facies variations were not considered. This current study, however, integrates modem geological concepts with historical production datato describe the remaining production data to describe the remainingrecompletion and infill-drilling potentials and to evaluate the feasibility ofenhanced recovery projects.
Geologic Setting
The Sansinena oil field is one of several major oil accumulations on thenortheast margin of the Los Angeles basin. Fig. 1 shows the location of theSansinena field, which is on trend with the Whittier and giant Brea-Olindafields. The Sansinena field consists of numerous stratigraphically andstructurally controlled sandstone reservoirs within the Upper Miocene Puenteand the Lower Pliocene Repetto formations (Fig. 2). Coarse-grainedsiliciclastic sediments form a sequence of coalesced submarine fans and feederchannel deposits that are interbedded and encased within thick, deepwatershales. The sands were derived from the highlands of the San Gabriel mountainsto the northeast and were transported across alluvial and shallow marineterrains to the shelf/slope break controlled by the Whittier fault uplift. Thestratigraphic thickness of the Upper Miocene/Lower Pliocene compositestratigraphic section exceeds 12,000 ft [3658 m] in this portion of the basin. The section is complicated by regional and local unconformities. These complexstratigraphic relationships appear to be controlled by syndepositional wrenchfault tectonism along the Whittier fault system. Continued fault deformationthrough the Pleistocene Age and even recent times has further obscured thesestratigraphic relationships. The entire section dips steeply toward the basincenter on the southwest flank of the Whittier fault trend. Thispostdepositional tilting was caused by rapid basin subsidence coupled withuplift along the Whittier fault. Oil accumulations in the Puente/Repetto sandsare trapped along the Whittier fault by a combination of fault truncation andupdip pinchout of submarine fan sands and by local anticlinal flexuring of thearea as a result of Whittier fault movement. Fig. 2 shows the stratigraphy andproducing zones in the eastern portion of the producing zones in the easternportion of the Sansinena field. The Miocene section of the Los Angeles basin issubdivided into a number of biostratigraphic divisions based on foraminiferalwork. Oil production from the Upper Miocene section in the Sansinena fieldcomes predominantly from Divs. A, C, and D sands. The A-10 North pool is thelargest and most productive pool in the eastern portion of the field.
The A-10 North Pool
The A-10 North pool is a northeast/ southwest-trending submarine channelcomplex with a variable internal fill of sands, silts, and shales. The channelis truncated updip (Fig. 3) by the frontal reverse fault of the Whittier faultsystem.
JPT
P. 909
