Drilling and completion of horizontal wells in any formation is often challenging. However, typical challenges are magnified in the diatomaceous Opal A Monterey Formation in the Belridge Field of Central California. Soft formations tend to be enlarged during wiper trips, while cuttings bed deposition, lost circulation, and gas migration can complicate cementing. Because of these challenges, horizontal well cement jobs in this field may not have been capable of providing effective containment of proppant fracture treatments within the target zones. Each well has several subzones, and each subzone is fractured separately. However, due to lack of containment, production results have been difficult to interpret because determining which fracture stage is producing the fluids is impossible. This paper outlines the methods used to greatly improve zonal isolation between subzones. Reducing the number of wiper trips and improved drilling fluid, directional drilling techniques, and tools helped provide improved hole geometry without cutting bed accumulation. Fully automated foamed cementing technology was applied to help achieve complete zonal isolation and full cement returns to the surface. Cement jobs were evaluated with special logging techniques and leading edge cement-bone logging tools with nontraditional interpretation algorithms. Finally, tracer materials were incorporated in proppant fracture treatments to verify that fractures were maintained in the appropriate zones. Detailed descriptions and actual job data are provided to document the significant improvements in drilling horizontal wells in this challenging field.